Leetcode:Reverse Words in a String

Description:

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.

分析： 根据题目中的解释，基本就可以弄清这个题目的基本要求. 翻转字符串中的单词。 基本想法是用stack存所有的单词，然后重建字符串。

代码中的注释有分析，后面那个runtime error告诉我们几种极端测试检查是需要的！

然后注意那个常用trick，在处理一些边界条件时还是很有用的 

class Solution {
public:
    void reverseWords(string &s) {
        stack<string> rec;
        if(s.empty()) return;
        //这里有这个trick,在最后加一个标志字符，象征结束，免得还要在后面循环结束后，看是否还有
        //单词没有加入stack中,常用trick
        s = s+" ";
        string onec;
        int startind = -1;
        for(int i=0;i<s.size();i++)
        {
            if(s[i]==' ' && startind==-1) continue;
            else if(s[i]==' '){
                onec.assign(s.begin()+startind,s.begin()+i);
                rec.push(onec);
                onec.clear();
                startind = -1;
                continue;
            }
            else if (startind==-1) startind=i;
            
        }
        s="";
        //必须判断rec是否为空，否则会runtime error，因为后面的删除操作
        if(rec.empty()) return;
        while(!rec.empty())
        {
            s = s+ rec.top()+" ";
            rec.pop();
        }
        s.erase(s.end()-1);
        
    }
};