Reverse String II — C++

// Time:  O(n)
// Space: O(1)

class Solution {
public:
    string reverseStr(string s, int k) {
        for (int left = 0; left < s.size(); left += 2 * k) {
            for (int i = left, j = min(left + k - 1, static_cast<int>(s.size()) - 1);
                 i < j; ++i, --j) {
                swap(s[i], s[j]);
            }
        }
        return s;
    }
};