LeetCode 186. Reverse Words in a String II

原题链接在这里：https://leetcode.com/problems/reverse-words-in-a-string-ii/

题目：

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

Example:

Input:  ["t","h","e"," ","s","k","y"," ","i","s"," ","b","l","u","e"]
Output: ["b","l","u","e"," ","i","s"," ","s","k","y"," ","t","h","e"]

Note: 

• A word is defined as a sequence of non-space characters.
• The input string does not contain leading or trailing spaces.
• The words are always separated by a single space.

Follow up: Could you do it in-place without allocating extra space?

题解：

input变成 char array. 

先reverse 全部 char array. 从头往后走，遇到空格或者array尾部，就reverse中间的单词.

Note: Do NOT forget i == n, reverse.

Time Compelxity: O(s.length()). Space: O(1).

AC Java:

public class Solution {
    public void reverseWords(char[] s) {
        if(s == null || s.length == 0){
            return;
        }
        reverse(s, 0, s.length-1);
        
        int lo = 0;
        for(int i = 1; i<=s.length; i++){
            if(i == s.length || s[i] == ' '){
                reverse(s, lo, i-1);
                lo = i+1;
            }
        }
    }
    
    private void reverse(char [] s, int i, int j){
        while(i < j){
            swap(s, i++, j--);
        }
    }
    
    private void swap(char [] s, int i , int j){
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
    }
}

类似Reverse Words in a String. 

跟上Reverse Words in a String III, Rotate Array.