leetcode[151]Reverse Words in a String

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.

class Solution {
public:
void reverseWords(string &s) {
    if(s.empty())return;
    string tmp="";
    string str="";
    int i=0;
    while(i<s.length())
    {
        while(s[i]==' '&&i<s.length())i++;
        if(s[i]!=' '&&i<s.length())
        {
            while(s[i]!=' '&&i<s.length())tmp=s[i++]+tmp;
            str+=(tmp+' ');
            tmp="";
        }
    }
    reverse(str.begin(),str.end());
    int n=str[0]==' '?1:0;
    s=str.substr(n,str.size());
    return;
}
};