541. Reverse String II

Description

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"

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

Solution

/**
 * @param {string} s
 * @param {number} k
 * @return {string}
 */
var reverseStr = function(s, k) {
    "use strict";
    function reverse(ary, i, j) {
        for (; i < j && i < ary.length; i++, j--) {
            let tmp = ary[i];
            ary[i] = ary[j];
            ary[j] = tmp;
        }
    }
    let result = s.split("");
    let i = k;
    while (true) {
        reverse(result, i-k, i-1);
        if (i >= s.length) {
            break;
        }
        i += 2*k;
    }
    return result.join("");
};