思路：两次翻转，先整体翻转，再局部翻转
    reverse/sort/vector的构造函数等，第二项不包括在内
    string也有begin(),end()两个迭代器

    1.     string ReverseSentence(string str) {
    2.         reverse(str.begin(),str.end());
    3.         int p1 = 0, p2 = 0, len = str.length();
    4.         while(p1 < len && p2 < len){
    5.             while(p1 < len && str[p1] == ' ')p1++;
    6.             p2 = p1;
    7.             while(p2 < len && str[p2] != ' ')p2++;
    8.             reverse(str.begin()+p1, str.begin()+p2);
    9.             p1 = p2;
    10.         }
    11.         return str;
    12.     }
    13. //第二种，自己写reverse
    14.     string ReverseSentence(string str) {
    15.         int len = str.length();
    16.         if(len <= 1)return str;
    17.         Reverse(str,0,len);
    18.         int p1 = 0, p2 = 0;
    19.         while(p1 < len && p2 < len){
    20.             while(p1 < len && str[p1] == ' ')p1++;
    21.             p2 = p1;
    22.             while(p2 < len && str[p2] != ' ')p2++;
    23.             Reverse(str, p1, p2);
    24.             p1 = p2;
    25.         }
    26.         return str;
    27.     }
    28.     void Reverse(string &str, int start, int end){
    29.         int len = end - start;
    30.         for(int i = 0;i < len / 2;i++){
    31.             swap(str[start + i], str[start + len - i - 1]);
    32.         }
    33.     }