左旋转字符串

class Solution {
public:
    string LeftRotateString(string str, int n) {
        if(str.length() == 0)
            return "";
        n = n % str.length();
        string left = str.substr(n,str.length()-n);
        string right = str.substr(0,n);
        string st = left + right;
        return st;
    }
};