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

For example,

Given s = "the sky is blue",

return "blue is sky the".

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.

思路：依次遍历输入string的每一个字符，

        若当前字符是空格，则循环继续；否则，读取一个单词，并将其加入新string的头部；重复上述步骤。

        将新string的值赋给string。

 1 void reverseWords( string &s ) { 2     string rStr = ""; 3     int size = s.length(); 4     for( int i = 0; i < size; ++i ) { 5         if( s[i] == ‘ ‘ ) { continue; } 6         int start = i; 7         while( ++i < size && s[i] != ‘ ‘ ) { ; } 8         if( rStr.empty() ) { 9             rStr = s.substr( start, i-start );10         } else {11             rStr = s.substr( start, i-start ) + " " + rStr;12         }13     }14     s = rStr;15     return;16 }