[LeetCode] 557. Reverse Words in a String III

Problem:

Given a string, you need to reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.

Example 1:

Input: "Let's take LeetCode contest"
Output: "s'teL ekat edoCteeL tsetnoc"

Note: In the string, each word is separated by single space and there will not be any extra space in the string.

Solution:

class Solution {
public:
    string reverseWords(string s) {
        reverse(s.begin(), s.end());           // s = "tsetnoc edoCteeL ekat s'teL"
        int lastspace = findLastSpaceIndex(s); // 定位最后一个空格的下标
        if (lastspace == -1) return s;          // 若不存在空格，则直接返回reverse后的串
        string res = "";
        while (s.length() > 0) {
            res += s.substr(lastspace + 1);    // 添加最后一个空格后的所有字符 s'teL
            res += " ";                        // 追加空格
            s = s.substr(0, lastspace);        // 去掉已经添加的部分 s = "tsetnoc edoCteeL ekat"
            lastspace = findLastSpaceIndex(s);
            if (lastspace == -1) break;
        }
        res += s;
        return res;
    }
    
    // 定位最后一个空格的下标，不存在返回-1
    int findLastSpaceIndex(string s) {
        int index = -1;
        for (int i = 0; i < s.length(); i++) {
            if (s[i] == ' ')
                index = i;
        }
        return index;
    }
};