541. Reverse String II

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.

Example:

Input: s = "abcdefg", k = 2
Output: "bacdfeg"

Restrictions:

1. The string consists of lower English letters only.

2. Length of the given string and k will in the range [1, 10000]

string reverseStr(string s, int k) { // time: O(n); space: O(n)
    int n = s.length();
    string res = s;
    for (int start = 0; start < n; start += 2 * k) {
        int i = start, j = min(start + k - 1, n - 1);
        while (i < j) {
            swap(res[i++], res[j--]);
        }
    }
    return res;
}