JS基础算法题（一）

数组字符串API

记录在FreeCodeCamp中遇到的Javascript基础算法题

1.翻转字符串

先把字符串转化成数组，再借助数组的reverse方法翻转数组顺序，最后把数组转化成字符串

1. function reverseString(str) { 

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2. var arr = str.split(""); 

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3. var reverseArr = arr.reverse(); 

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4. str = reverseArr.join(''); 

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5. return str; 

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6. } 

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7.  

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8. reverseString("hello"); //olleh 

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用到的方法：String.split();Array.join();Array.reverse()

2.计算整数的阶乘

如果用字母n来代表一个整数，阶乘代表着所有小于或等于n的整数的乘积。

1. function factorialize(num) { 

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2. var result =1; 

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3. for(var i=1;i<=num;i++){ 

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4. result*=i; 

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5. } 

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6. return result; 

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7. } 

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8.  

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9. factorialize(5);//5*4*3*2*1=120 

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3.回文字符串

如果一个字符串忽略标点符号、大小写和空格，正着读和反着读一模一样，那么这个字符串就是palindrome(回文)。

1. function palindrome(str) { 

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2. str = str.replace(/[^\w]/gi, "").replace(/_/g, "");  

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3. str = str.toLowerCase(); 

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4. var reverseStr = str.split("").reverse().join(''); 

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5. if (str == reverseStr) { 

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6. return true; 

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7. } else { 

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8. return false; 

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9. } 

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10. } 

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11.  

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12. palindrome("eye"); 

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涉及知识点：String.replace();String.toLowerCase();正则表达式

4.找出最长的单词

在句子中找出最长的单词，并返回它的长度。

1. function findLongestWord(str) { 

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2. str = str.split(" "); 

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3. //这里我用了一个比较麻烦的方法，冒泡排序;简便方法可以看第6题 

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4. for (var i = 0; i < str.length; i++) { 

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5. for (var j = 0; j < str.length - 1 - i; j++) { 

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6. if (str[j + 1].length > str[j].length) { 

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7. var temp = str[j]; 

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8. str[j] = str[j + 1]; 

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9. str[j + 1] = temp; 

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10. } 

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11. } 

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12. } 

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13. return str[0].length; 

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14. } 

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15.  

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16. findLongestWord("The quick brown fox jumped over the lazy dog"); 

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涉及知识：String.split();String.length;排序算法

5.字符串首字母大写

句中单词首字母大写。确保字符串的每个单词首字母都大写，其余部分小写。像'the'和'of'这样的连接符同理。

1. function titleCase(str) { 

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2. var strArr = str.toLowerCase().split(" "); 

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3. for(var i=0;i<strArr.length;i++){ 

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4. var first = strArr[i].charAt(0);//返回指定位置的字符,标点符号也包括在内 

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5. strArr[i] = strArr[i].replace(first,function(first){ 

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6. return first.toUpperCase(); 

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7. }); 

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8. } 

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9. str = strArr.join(" "); 

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10. return str; 

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11. } 

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12.  

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13. titleCase("I'm a little tea pot"); 

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涉及知识：String.charAt();String.split();String.toUpperCase()

6.找出多个数组中的最大数

1. function largestOfFour(arr) { 

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2. var largestArr = []; 

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3. for(var i=0;i<arr.length;i++){ 

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4. var largest = 0; 

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5. for(var j=0;j<arr[i].length;j++){ 

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6. largest = arr[i][j]>largest?arr[i][j]:largest; 

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7. } 

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8. largestArr.push(largest); 

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9. } 

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10. return largestArr; 

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11. } 

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12.  

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13. largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]); //[5,27,39,1001] 

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涉及知识：三元运算符;比较操作符

7.检查字符串结尾

判断一个字符串(str)是否以指定的字符串(target)结尾。

1. function confirmEnding(str, target) { 

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2. //substr()在字符串中抽取从 start 下标开始的指定数目的字符 

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3. if(target == str.substr(str.length - target.length, target.length)){ 

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4. return true; 

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5. }else{ 

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6. return false; 

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7. } 

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8. } 

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9.  

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10. confirmEnding("Bastian", "n");//true 

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11. confirmEnding("He has to give me a new name", "me");//true 

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涉及知识：String.substr();

8.重复输出字符串

重复一个指定的字符串 num次，如果num是一个负数则返回一个空字符串。

1. function repeat(str, num) { 

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2. var res = ""; 

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3. for(var i=0; i<num; i++){ 

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4. res += str; 

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5. } 

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6. return res; 

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7. } 

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8.  

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9. repeat("abc", 3); 

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9.截断字符

如果字符串的长度比指定的参数num长，则把多余的部分用...来表示。

切记，插入到字符串尾部的三个点号也会计入字符串的长度。

但是，如果指定的参数num小于或等于3，则添加的三个点号不会计入字符串的长度。

1. function truncate(str, num) { 

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2. var extra=""; 

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3. if(str.length>num){ 

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4. if(num>3){ 

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5. extra = str.slice(0,num-3)+"..."; 

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6. }else{ 

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7. extra = str.slice(0,num)+"..."; 

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8. } 

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9. return extra; 

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10. } 

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11. return str; 

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12. } 

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13.  

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14. truncate("A-tisket a-tasket A green and yellow basket", 11);//A-tisket... 

    ---

涉及知识：String.slice()

10.截断数组

返回一个数组被截断n个元素后还剩余的元素，截断从索引0开始

1. function slasher(arr, howMany) { 

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2. arr.splice(0,howMany); 

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3. return arr; 

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4. } 

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5.  

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6. slasher([1, 2, 3], 2); 

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涉及知识：Array.splice()