151. Reverse Words in a String - Medium-优快云博客

本文介绍了一种算法，用于将输入字符串按单词进行反转，同时去除多余空格并保持单词间的单个空格间隔。通过实例展示了如何在不使用额外空间的情况下，实现字符串中单词的反转操作。

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.

two pointers

time: O(n), space: O(1)

class Solution {
    public String reverseWords(String s) {
        if(s == null || s.length() == 0) {
            return "";
        }
        char[] str = reverse(s.toCharArray(), 0, s.length() - 1);
        int start = 0, end = 0;
        for(int i = 0; i < str.length; i++) {
            if(str[i] != ' ') {
                str[end++] = str[i];
            } else if(i > 0 && str[i] == ' ' && str[i - 1] != ' ') {
                reverse(str, start, end - 1);
                str[end++] = ' ';
                start = end;
            }
        }
        reverse(str, start, end - 1);   // last word
        
        return end > 0 && str[end - 1] == ' ' ? new String(str, 0, end - 1) : new String(str, 0, end);
    }
    
    private char[] reverse(char[] c, int i, int j) {
        while(i < j) {
            char tmp = c[i];
            c[i] = c[j];
            c[j] = tmp;
            i++;
            j--;
        }
        return c;
    }
}