剑指offer42 左旋转字符串

自己想的一个新的写法，如果不排除length=0的情况，下面那个while是死循环

class Solution {
public:
    string LeftRotateString(string str, int n) {
        int length = str.length();
        if(length == )
            return str;
        while(length <= n)
             n = n - length;
        Rotate(str,,length-);
        Rotate(str,,length--n);
        Rotate(str,length-n,length-);
        return str;
    }
    void Rotate(string &str,int start,int end){
        for(int i = start;i <= (start + end)/;i++){
            char tmp = str[i];
            str[i] = str[start+end-i];
            str[start+end-i] = tmp;
        }
    }
};