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​​".

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

​​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:

/*
Input:  "   a   b "
Output: "b   a"
Expected:   "b a"

    void reverseWords(string &s) {
        
        int l=0,r=s.size()-1;
        int k;
        
        while(s[l]==' ') l++;
        while(s[r]==' ') r--;
        if(l>r)
        { 
            s="";
            return ;
        }
        reverse(s,l,r);
        for(int i=l;i<=r;i++)
        {
            while(s[i]==' ')
                i++;
            k=i;
            while(i<=r&&s[i]!=' ')
                i++;
            i--;
            reverse(s,k,i);
        }
        s=s.substr(l,r-l+1);
    }
    
    void reverse(string &str,int s,int e)
    {
        char temp;
        for(int i=s,j=e;i<=j;i++,j--)
        {
            temp=str[i];
            str[i]=str[j];
            str[j]=temp;
        }
    }
    */
    
    //用vector处理，从尾开始放，每次放入之前逆转
     void reverseWords(string &s) {
        
        string ans;
        int i=s.size()-1;
        
        while(i>=0)
        {
            while(i>=0&&s[i]==' ')
                i--;
            if(i<0)
                break;
            if(ans.size()>0)
                ans+=' ';
            string tmp;
            for(;i>=0&&s[i]!=' ';i--)
                tmp+=s[i];
            reverse(tmp.begin(),tmp.end());
            ans+=tmp;
        }
        s=ans;
    }
};