左旋转字符串

class Solution {
public:
    string reverse(string str,int i,int j)
        {
        while(i<j)
             {
             char tmp=str[j];
            str[j]=str[i];
            str[i]=tmp;
            i++;j--;
         }
        return str;
    }
    string LeftRotateString(string str, int n) {
        string s;
       if(str.empty())
           return s;
        if(n>str.size())return s;
        s=reverse(str,0,str.size()-1);
        s=reverse(s,0,s.size()-n-1);
        s=reverse(s,s.size()-n,s.size()-1);
        return s;
    }
};