leetcode 796. Rotate String

We are given two strings, A and B.

A shift on A consists of taking string A and moving the leftmost character to the rightmost position. For example, if A = 'abcde', then it will be 'bcdea' after one shift on A. Return True if and only if A can become B after some number of shifts on A.

Example 1:
Input: A = 'abcde', B = 'cdeab'
Output: true

Example 2:
Input: A = 'abcde', B = 'abced'
Output: false

class Solution {
public:
    bool rotateString(string A, string B) {
        if (A.size() != B.size()) return false;
        string ds = A + A;
        int len = A.size();
        for (int i = 0; i < ds.size()/2 ; ++i) {
            string s = ds.substr(i, len);
            if (s == B) return true;
        }
        return false;
    }
};