LeetCode Reverse Words in a String(反转字符串)

题目要求：

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

For example,
Given s = "the sky is blue",
return "blue is sky the".

click to show clarification.

Clarification:

• What constitutes a word?
  A sequence of non-space characters constitutes a word.
• Could the input string contain leading or trailing spaces?
  Yes. However, your reversed string should not contain leading or trailing spaces.
• How about multiple spaces between two words?
  Reduce them to a single space in the reversed string

我是用c++ 写的：

class Solution {
public:
  void reverseWords(string &s) {
    size_t idx_start = 0;
    size_t idx_end;
    //erase the space at head and trail
    idx_start = s.find_first_not_of(" ");
    s.erase(0, idx_start);
    idx_end = s.find_last_not_of(" ");
    s.erase(idx_end + 1);
    reverse(s.begin(), s.end());
    idx_start = 0;
    string ret;
    string tmp;
    while(true)
    {
      tmp.clear();
      idx_end = s.find(' ', idx_start);
      if(idx_end == string::npos)
        break;
      //deal the sitution of contiguous spaces like "   "
      if(s[idx_start] == ' ')
      {
        idx_start = idx_end + 1;
        continue;
      }
      tmp = s.substr(idx_start, idx_end - idx_start);
      reverse(tmp.begin(), tmp.end());
      ret.append(tmp);
      ret.append(" ");
      idx_start = idx_end + 1;
    }
    //reverse the last word
    tmp = s.substr(idx_start);
    reverse(tmp.begin(), tmp.end());
    ret.append(tmp);
    s = ret;
  }
};