[LeetCode] 151. Reverse Words in a String

Given an input string, reverse the string word by word.

Example 1:

Input: "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: "  hello world!  "
Output: "world! hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: "a good   example"
Output: "example good a"
Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Note:

A word is defined as a sequence of non-space characters.
Input string may contain leading or trailing spaces. However, your reversed string should not contain leading or trailing spaces.
You need to reduce multiple spaces between two words to a single space in the reversed string.

Follow up:

For C programmers, try to solve it in-place in O(1) extra space.

翻转字符串里的单词。题目即是题意。有两种corner case需要排除，一种是input前后多余的空格，一种是单词之间的多余空格。思路是先把input字符串分开成数组，再将数组reverse，最后将数组的每个元素拼接成字符串。

时间O(n)

空间O(n)

Java实现 - 正则表达

"s" is a regex class for any kind of whitespace (space, tab, newline, etc). Since Java uses "" as an escape character in strings (e.g. for newlines: " "), we need to escape the escape character ;-) So it becomes "\s". The "+" means one or more of them.

 1 class Solution {
 2     public String reverseWords(String s) {
 3         // corner case
 4         if (s == null || s.length() == 0) {
 5             return s;
 6         }
 7 
 8         // normal case
 9         StringBuilder sb = new StringBuilder();
10         // trim 前后 skip 中间的回车 空格之类的东西
11         String[] words = s.trim().split("\s+");
12         for (int i = words.length - 1; i >= 0; i--) {
13             sb.append(words[i] + " ");
14         }
15         return sb.toString().trim();
16     }
17 }

Java拼接实现

这种思路是trim完input之后，从后往前开始遍历input，用双指针卡住单词。先移动其中一个指针，当遇到空格的时候就需要截取单词了；将单词加入结果集之后记得要跳过单词之间的空格。

 1 class Solution {
 2     public String reverseWords(String s) {
 3         s = s.trim(); // 删除首尾空格
 4         int j = s.length() - 1;
 5         int i = j;
 6         StringBuilder res = new StringBuilder();
 7         while (i >= 0) {
 8             while (i >= 0 && s.charAt(i) != ' ') {
 9                 i--; // 搜索首个空格
10             }
11             res.append(s.substring(i + 1, j + 1) + " "); // 添加单词
12             while (i >= 0 && s.charAt(i) == ' ') {
13                 i--; // 跳过单词间空格
14             }
15             j = i; // j 指向下个单词的尾字符
16         }
17         return res.toString().trim(); // 转化为字符串并返回
18     }
19 }

JavaScript实现

 1 /**
 2  * @param {string} s
 3  * @return {string}
 4  */
 5 var reverseWords = function (s) {
 6     return s
 7         .split(' ')               //create an array of words separated based by spaces
 8         .filter(string => string) //remove empty strings to take care of extra whitespace
 9         .reverse()                //reverse the array of words
10         .join(' ');               //join the words back together with spaces inbetween
11 };