字符串操作大全：面试准备和日常编码所需一文打尽！

时间：2022-05-11 07:09

技术图片
图源：unsplash

字符串是一系列字符，由常数或变量构成。它是编程语言中必不可少的数据类型。本文中将重点关注JavaScript字符串操作，但其原理和算法也可应用于其他语言。

参加技术面试时，面试官常常会关注以下内容：

· 编程技术
· 语言能力
· 解题技巧

本文不仅可以让你成功通过技术面试，对日常编码也很有用。代码要点格式中，我们列出了JavaScript字符串的几点重要特性，这是编程技能的基础。其中包括存在了20年的属性和方法，也涵盖ES2021中的特性。如有不清楚之处，可以有针对性地查漏补缺。JavaScript编程语言可以解决许多应用问题。这些算法或其变体，经常出现在真实的面试场景中。

技术图片

字符串属性和方法

字符串用于表示和操作字符序列。字符串属性和方法有很多。以下是可供参考的代码示例，包括ES2020中的“matchAll”和ES2021中的“replaceAll”。

const str = Today is a nice day! ;  
      console.log(str.length); // 20
      console.log(str[2]); // "d"
      console.log(typeof str); // "string"
      console.log(typeof str[2]); // "string"
      console.log(typeofString(5)); //"string"
      console.log(typeofnewString(str)); //"object"
      console.log(str.indexOf( is )); // 6
      console.log(str.indexOf( today )); // -1
      console.log(str.includes( is )); // true
      console.log(str.includes( IS )); // false
      console.log(str.startsWith( Today )); // true
      console.log(str.endsWith( day )); // false
      console.log(str.split(   )); // ["Today", "is", "a", "nice","day!"]
      console.log(str.split(  )); // ["T", "o", "d", "a","y", " ", "i", "s", " ","a", " ", "n", "i", "c","e", " ", "d", "a", "y","!"]
      console.log(str.split( a )); // ["Tod", "y is ", " nice d","y!"]
      console.log(str +1+2); // "Today is a nice day!12"
      console.log(str + str); // "Today is a nice day!Today is a niceday!"
      console.log(str.concat(str)); // "Today is a nice day!Today is a niceday!"
      console.log(str.repeat(2)); // "Today is a nice day!Today is a nice day!"
      console.log( abc < bcd ); // true
      console.log( abc .localeCompare( bcd )); // -1
      console.log( a .localeCompare( A )); // -1
      console.log( a .localeCompare( A , undefined, { numeric: true })); // -1
      console.log( a .localeCompare( A , undefined, { sensitivity:  accent  })); // 0
      console.log( a .localeCompare( A , undefined, { sensitivity:  base  })); // 0
      console.log( a .localeCompare( A! , undefined, { sensitivity:  base , ignorePunctuation: true })); // 0
      console.log( abc .toLocaleUpperCase()); // "ABC"
      console.log(str.padStart(25,  * )); // "*****Todayis a nice day!"
      console.log(str.padEnd(22,  ! )); // "Today is anice day!!!"
      console.log(    middle  .trim().length); // 6
      console.log(    middle  .trimStart().length); // 8
      console.log(    middle  .trimEnd().length); // 9
      console.log(str.slice(6, 8)); // "is"
      console.log(str.slice(-4)); // "day!"
      console.log(str.substring(6, 8)); // "is"
      console.log(str.substring(-4)); // "Today is a nice day!"
      console.log( a .charCodeAt()); // 97
      console.log(String.fromCharCode(97)); // "a"
      console.log(str.search(/[a-c]/)); // 3
      console.log(str.match(/[a-c]/g)); // ["a", "a", "c", "a"]
      console.log([...str.matchAll(/[a-c]/g)]);
      // [Array(1), Array(1), Array(1), Array(1)]
      // 0: ["a", index: 3, input: "Today is a nice day!",groups: undefined]
      // 1: ["a", index: 9, input: "Today is a nice day!",groups: undefined]
      // 2: ["c", index: 13, input: "Today is a niceday!", groups: undefined]
      // 3: ["a", index: 17, input: "Today is a niceday!", groups: undefined]
      console.log([... test1test2 .matchAll(/t(e)(st(d?))/g)]);
      // [Array(4), Array(4)]
      // 0: (4) ["test1", "e", "st1","1", index: 0, input: "test1test2", groups: undefined]
      // 1: (4) ["test2", "e", "st2","2", index: 5, input: "test1test2", groups: undefined]
      console.log(str.replace( a ,  z )); // Todzy is anice day!
      console.log(str.replace(/[a-c]/,  z )); // Todzy is anice day!
      console.log(str.replace(/[a-c]/g,  z )); // Todzy is znize dzy!
      console.log(str.replaceAll( a ,  z )); // Todzy is znice dzy!
      console.log(str.replaceAll(/[a-c]/g,  z )); // Todzy is znize dzy!
      console.log(str.replaceAll(/[a-c]/,  z )); // TypeError:String.prototype.replaceAll called with a non-global RegExp argument

技术图片

映射和集合

对于字符串操作，我们需要在某处存储中间值。数组、映射和集合都是需要掌握的常用数据结构，本文主要讨论集合和映射。

集合

Set是存储所有类型的唯一值的对象。以下是供参考的代码示例，一目了然。

const set =newSet( aabbccdefghi ); 
                       console.log(set.size); // 9
                       console.log(set.has( d )); // true
                       console.log(set.has( k )); // false
                       console.log(set.add( k )); // {"a", "b", "c", "d","e" "f", "g", "h", "i","k"}
                       console.log(set.has( k )); // true
                       console.log(set.delete( d )); // true
                       console.log(set.has( d )); // false
                       console.log(set.keys()); // {"a", "b", "c","e" "f", "g", "h", "i","k"}
                       console.log(set.values()); // {"a", "b", "c","e" "f", "g", "h", "i","k"}
                       console.log(set.entries()); // {"a" => "a","b" => "b", "c" => "c","e" => "e",
                       // "f"=> "f", "g" => "g", "h" =>"h"}, "i" => "i", "k" =>"k"}
                       const set2 =newSet();
                       set.forEach(item => set2.add(item.toLocaleUpperCase()));
                       set.clear();
                       console.log(set); // {}
                       console.log(set2); //{"A", "B", "C", "E", "F","G", "H", "I", "K"}
                       console.log(newSet([{ a: 1, b: 2, c: 3 }, { d: 4, e: 5 }, { d: 4, e: 5 }]));
                       // {{a: 1, b: 2,c: 3}, {d: 4, e: 5}, {d: 4, e: 5}}
                       const item = { f: 6, g: 7 };
                       console.log(newSet([{ a: 1, b: 2, c: 3 }, item, item]));
                       // {{a: 1, b: 2,c: 3}, {f: 6, g: 7}}

映射

映射是保存键值对的对象。任何值都可以用作键或值。映射会记住键的原始插入顺序。以下是供参考的代码示例：

const map =newMap();   
      console.log(map.set(1,  first )); // {1 =>"first"}
      console.log(map.set( a ,  second )); // {1 =>"first", "a" => "second"}
      console.log(map.set({ obj:  123  }, [1, 2, 3]));
      // {1 => "first", "a" =>"second", {obj: "123"} => [1, 2, 3]}
      console.log(map.set([2, 2, 2], newSet( abc )));
      // {1 => "first", "a" => "second",{obj: "123"} => [1, 2, 3], [2, 2, 2] => {"a","b", "c"}}
      console.log(map.size); // 4
      console.log(map.has(1)); // true
      console.log(map.get(1)); // "first"
      console.log(map.get( a )); // "second"
      console.log(map.get({ obj:  123  })); // undefined
      console.log(map.get([2, 2, 2])); // undefined
      console.log(map.delete(1)); // true
      console.log(map.has(1)); // false
      const arr = [3, 3];
      map.set(arr, newSet( xyz ));
      console.log(map.get(arr)); // {"x", "y", "z"}
      console.log(map.keys()); // {"a", {obj: "123"}, [2, 2,2], [3, 3]}
      console.log(map.values()); // {"second", [1, 2, 3], {"a","b", "c"}, {"x", "y", "z"}}
      console.log(map.entries());
      // {"a" => "second", {obj: "123"}=> [1, 2, 3], [2, 2, 2] => {"a", "b", "c"},[3, 3] => {"x", "y", "z"}}
      const map2 =newMap([[ a , 1], [ b , 2], [ c , 3]]);
      map2.forEach((value, key, map) => console.log(`value = ${value}, key = ${key}, map = ${map.size}`));
      // value = 1, key = a, map = 3
      // value = 2, key = b, map = 3
      // value = 3, key = c, map = 3
      map2.clear();
      console.log(map2.entries()); // {}

技术图片

应用题

面试中有英语应用题，我们探索了一些经常用于测试的算法。

等值线

等值线图是指所含字母均只出现一次的单词。

· dermatoglyphics (15个字母)
· hydropneumatics (15个字母)
· misconjugatedly (15个字母)
· uncopyrightable (15个字母)
· uncopyrightables (16个字母)
· subdermatoglyphic (17个字母)

如何写一个算法来检测字符串是否是等值线图？有很多方法可以实现。可以把字符串放在集合中，然后自动拆分成字符。由于集合是存储唯一值的对象，如果它是一个等值线图，它的大小应该与字符串长度相同。

/**
    * An algorithm to verify whethera given string is an isogram
    * @param {string} str The string to be verified
    * @return {boolean} Returns whether it is an isogram
    */
   functionisIsogram(str) {
    if (!str) {
       returnfalse;
    }

        const set =newSet(str);
    return set.size=== str.length;
   }

以下是验证测试:

console.log(isIsogram(  )); // false 
                            console.log(isIsogram( a )); // true
                            console.log(isIsogram( misconjugatedly )); // true
                            console.log(isIsogram( misconjugatledly )); // false

全字母短句

全字母短句是包含字母表中所有26个字母的句子，不分大小写。理想情况下，句子越短越好。以下为全字母短句:

· Waltz, bad nymph, for quick jigs vex. (28个字母)
· Jived fox nymph grabs quick waltz. (28个字母)
· Glib jocks quiz nymph to vex dwarf. (28个字母)
· Sphinx of black quartz, judge my vow. (29个字母)
· How vexingly quick daft zebras jump! (30个字母)
· The five boxing wizards jump quickly. (31个字母)
· Jackdaws love my big sphinx of quartz. (31个字母)
· Pack my box with five dozen liquor jugs. (32个字母)
· The quick brown fox jumps over a lazy dog. (33个字母)

还有很多方法可以验证给定的字符串是否是全字母短句。这一次，我们将每个字母(转换为小写)放入映射中。如果映射大小为26，那么它就是全字母短句。

/**   
    * An algorithm to verify whethera given string is a pangram
    * @param {string} str The string to be verified
    * @return {boolean} Returns whether it is a pangram
    */
   functionisPangram(str) {
    const len = str.length;
    if (len <26) {
       returnfalse;
    }
               const map =newMap();
    for (let i =0; i < len; i++) {
       if (str[i].match(/[a-z]/i)) { // if it is letter a to z, ignoring the case
         map.set(str[i].toLocaleLowerCase(), true); // use lower case letter as a key
       }
    }
    return map.size===26;
   }

以下是验证测试:

console.log(isPangram(  )); // false 
                            console.log(isPangram( Bawds jog, flick quartz, vex nymphs. )); // true
                            console.log(isPangram( The quick brown fox jumped over the lazy sleepingdog. )); // true
                            console.log(isPangram( Roses are red, violets are blue, sugar is sweet,and so are you. )); // false

同构字符串

给定两个字符串s和t，如果可以替换掉s中的字符得到t，那么这两个字符串是同构的。s中的所有字符转换都必须应用到s中相同的字符上，例如，murmur与tartar为同构字符串，如果m被t替换，u被a替换，r被自身替换。以下算法使用数组来存储转换字符，也适用于映射。

/**
    * An algorithm to verify whethertwo given strings are isomorphic
    * @param {string} s The first string
    * @param {string} t The second string
    * @return {boolean} Returns whether these two strings are isomorphic
    */
   functionareIsomorphic(s, t) {
    // strings with different lengths are notisomorphic
    if (s.length !== t.length) {
         returnfalse;
    }
               // the conversion array
    const convert = [];
    for (let i =0; i < s.length; i++) {
         // if the conversioncharacter exists
         if (convert[s[i]]) {
             // apply the conversion and compare
             if (t[i] === convert[s[i]]) { // so far so good
                 continue;
             }
             returnfalse; // not isomorphic
         }
                   // set the conversion character for future use
         convert[s[i]] = t[i];
    }
               // these two strings are isomorphic since there are no violations
    returntrue;
   };

以下是验证测试:

console.log(areIsomorphic( atlatl ,  tartar )); // true
                                    console.log(areIsomorphic( atlatlp ,  tartarq )); // true
                                    console.log(areIsomorphic( atlatlpb ,  tartarqc )); // true
                                    console.log(areIsomorphic( atlatlpa ,  tartarqb )); // false

相同字母异构词

相同字母异构词是通过重新排列不同单词的字母而形成的单词，通常使用所有原始字母一次。从一个池中重新排列单词有很多种可能性。例如，cat的相同字母异构词有cat、act、atc、tca、atc和tac。我们可以添加额外的要求，即新单词必须出现在源字符串中。如果源实际上是actually，则结果数组是[“act”]。

/**
    * Given a pool to compose ananagram, show all anagrams contained (continuously) in the source
    * @param {string} source A source string to draw an anagram from
    * @param {string} pool A pool to compose an anagram
    * @return {array} Returns an array of anagrams that are contained by the source string
    */
   functionshowAnagrams(source, pool) {
    // if source is not long enough to hold theanagram
    if (source.length< pool.length) {
       return [];
    }
               const sourceCounts = []; // an array tohold the letter counts in source
    const poolCounts = []; // an array tohold the letter counts in pool
               // initialize counts for 26 letters to be 0
    for (let i =0; i <26; i++) {
       sourceCounts[i] =0;
       poolCounts[i] =0;
    }
               // convert both strings to lower cases
    pool = pool.toLocaleLowerCase();
    const lowerSource = source.toLocaleLowerCase();
               for (let i =0; i < pool.length; i++) {
        // calculatepoolCounts for each letter in pool, mapping a - z to 0 - 25
       poolCounts[pool[i].charCodeAt() -97]++;
    }
               const result = [];
    for (let i =0; i < lowerSource.length; i++) {
       // calculatesourceCounts for each letter for source, mapping a - z to 0 - 25
       sourceCounts[lowerSource[i].charCodeAt() -97]++;
       if (i >= pool.length-1) { // if source islong enough
         // if sourceCountsis the same as poolCounts
         if (JSON.stringify(sourceCounts) ===JSON.stringify(poolCounts)) {
           // save the found anagram, using the original source to make stringcase-preserved
           result.push(source.slice(i - pool.length+1, i +1));
         }
         // shift thestarting window by 1 index (drop the current first letter)
         sourceCounts[lowerSource[i - pool.length+1].charCodeAt() -97]--;
       }
    }
          // removeduplicates by a Set
    return [...newSet(result)];
   }

以下是验证测试:

console.log(showAnagrams( AaaAAaaAAaa ,  aa )); // ["Aa", "aa", "aA", "AA"]
                                        console.log(showAnagrams( CbatobaTbacBoat ,  Boat ));  //["bato", "atob", "toba", "obaT","Boat"]
                                        console.log(showAnagrams( AyaKkayakkAabkk ,  Kayak ));
                                        // ["AyaKk", "yaKka", "aKkay", "Kkaya","kayak", "ayakk", "yakkA"]

回文

回文是从前往后读和从后往前读读法相同的单词或句子。有很多回文，比如A，Bob，还有 “A man, a plan, a canal — Panama”。检查回文的算法分为两种。使用循环或使用递归从两端检查是否相同。下列代码使用递归方法：

/**
    * An algorithm to verify whethera given string is a palindrome
    * @param {string} str The string to be verified
    * @return {boolean} Returns whether it is a palindrome
    */
   functionisPalindrome(str) {
    functioncheckIsPalindrome(s) {
       // empty stringor one letter is a defecto palindrome
       if (s.length<2) {
         returntrue;
       }
                 if ( // if two ends notequal, ignoring the case
         s[0].localeCompare(s[s.length-1], undefined, {
           sensitivity:  base ,
         }) !== 0
       ) {
         returnfalse;
       }
                 // since two ends equal, checking the inside
       returncheckIsPalindrome(s.slice(1, -1));
    }

    // check whether it is a palindrome, removing noneletters and digits
    returncheckIsPalindrome(str.replace(/[^A-Za-z0-9]/g,   ));
   }

以下是验证测试:

console.log(isPalindrome( )); // true
console.log(isPalindrome( a )); // true
console.log(isPalindrome( Aa )); // true
console.log(isPalindrome( Bob )); // true
console.log(isPalindrome( Odd or even )); // false
console.log(isPalindrome( Never odd or even )); // true
console.log(isPalindrome( 02/02/2020 )); // true
console.log(isPalindrome( 2/20/2020 )); // false
console.log(isPalindrome( A man, a plan, a canal – Panama )); // true

回文面试题有很多不同的变形题，下面是一个在给定字符串中寻找最长回文的算法。

/**                                       
    * An algorithm to find thelongest palindrome in a given string
    * @param {string} source The source to find the longest palindrome from
    * @return {string} Returns the longest palindrome
    */
   functionfindLongestPalindrome(source) {
    // convert to lower cases and only keep lettersand digits
    constlettersAndDigits = source.replace(/[^A-Za-z0-9]/g,   );
    const str = lettersAndDigits.toLocaleLowerCase();
               const len = str.length;
              // empty string or one letter is a defecto palindrome
    if (len <2) {
       return str;
    }
               // the first letter is the current longest palindrome
    let maxPalindrome = lettersAndDigits[0];
               // assume that the index is the middle of a palindrome
    for (let i =0; i < len; i++) {
                 // try the case that the palindrome has one middle
       for (
         let j =1; // start with onestep away (inclusive)
         j < len &&// end with the len end (exclusive)
         i - j >= 0&&// cannot pass the start index (inclusive)
         i + j < len &&// cannot exceed end index (exclusive)
         Math.min(2 * i +1, 2 * (len - i) -1) > maxPalindrome.length; // potential max length should be longer than thecurrent length
         j++
       ) {
         if (str[i - j] !== str[i + j]) { // if j stepsbefore the middle is different from j steps after the middle
           break;
         }
         if (2 * j +1> maxPalindrome.length) { // if it is longerthan the current length
           maxPalindrome = lettersAndDigits.slice(i - j, i + j +1); // j steps before, middle, and j steps after
         }
       }
                 // try the case that the palindrome has two middles
       if (i < len -1&& str[i] === str[i +1]) { // if two middles are the same
         if (maxPalindrome.length<2) { // the string withtwo middles could be the current longest palindrome
           maxPalindrome = lettersAndDigits.slice(i, i +2);
         }
         for (
           let j =1; // start with one step away (inclusive)
           j < len -1&&// end with the len - 1 end (exclusive)
           i - j >= 0&&// cannot pass the start index (inclusive)
           i + j +1< len &&// cannot exceed end index (exclusive)
           Math.min(2 * i +2, 2 * (len - i)) > maxPalindrome.length; // potential max length should be longer than thecurrent length
           j++
         ) {
           if (str[i - j] !== str[i + j +1]) { // if j stepsbefore the left middle is different from j steps after the right middle
             break;
           }
           if (2 * j +2> maxPalindrome.length) { // if it is longer than the current length
             maxPalindrome = lettersAndDigits.slice(i - j, i + j +2); // j steps before, middles, and j steps after
           }
         }
       }
    }
    return maxPalindrome;
   }

以下是验证测试:

console.log(findLongestPalindrome(  )); // "" 
                                        console.log(findLongestPalindrome( abc )); // "a"
                                        console.log(findLongestPalindrome( Aabcd )); // "Aa"
                                        console.log(findLongestPalindrome( I am Bob. )); // "Bob"
                                        console.log(findLongestPalindrome( Odd or even )); // "Oddo"
                                        console.log(findLongestPalindrome( Never odd or even )); // "Neveroddoreven"
                                        console.log(findLongestPalindrome( Today is 02/02/2020. )); // "02022020"
                                        console.log(findLongestPalindrome( It is 2/20/2020. )); // "20202"
                                        console.log(findLongestPalindrome( A man, a plan, a canal – Panama )); // "AmanaplanacanalPanama"

熟能生巧。享受编码！