541. Reverse String II

class Solution {
public:
    string reverseStr(string s, int k) {
        for (size_t i = 0; i < s.length(); i += 2 * k) {
            int start = i;
            int end = min(i+k, s.length()) - 1;
            while (start < end)
                swap(s[start++], s[end--]);
        }
        return s;
    }
};