Problem

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: The string consists of lower English letters only. Length of the given string and k will in the range [1, 10000]

Pre analysis

Will group string into an array and return joined array with alternate reverse mechanism;

Another solution

Iterating over

    var reverseStr = function (s, k) {
        let len = s.length;
        let j = 2 * k;
        let i = 0;
        let res = "";
        function reverse(s, start, end) {
            let tmp = "";
            while (--end >= start) {
                tmp += s[end];
            }
            return tmp;
        }
        while (j < len) {
            res = res.concat(reverse(s, i, i + k), s.slice(i + k, j));
            i = j;
            j += 2 * k;
        }
        if (i < len) {
            let n = Math.min(len, i + k);
            res = res.concat(reverse(s, i, n));
            if (n < len) {
                res = res.concat(s.slice(n, len));
            }
        }

        return res;
    };