String - 186. Reverse Words in a String II

Reverse Words in a String II

Given an input character array, reverse the array word by word. A word is defined as a sequence of non-space characters.

The input character array does not contain leading or trailing spaces and the words are always separated by a single space.

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Problem Correction

Example

Example1

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

Example2

Input: "a b c"
Output: "c b a"

Challenge

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

思路：

因为题目要求只能在原数组操作，不能申请额外的空间，并且字母之间只有一个空格，所以使用翻转两次的方式，第一次翻转整个字符串，第二次翻转单个单词。

代码：

java：

public class Solution {
    /**
     * @param str: a string
     * @return: return a string
     */
    public char[] reverseWords(char[] str) {
        if (str == null || str.length == 0) return new char[]{};

        int len = str.length;
        // 1. reverse whole string
        reverse(str, 0, len - 1);

        // 2. reverse single word
        int start = 0, i = 0;
        for (;i < len; i++) {
            if (str[i] == ' ') {
                reverse(str, start, i - 1);
                start = i + 1;
            }
        }
        reverse(str,start, i-1);
        
        return str;
    }

    private void reverse(char[] s, int i, int j) {
        while( i < j) {
            char temp = s[i];
            s[i++] = s[j];
            s[j--] = temp;
        }
    }
}