541. Reverse String II #

Problem Statement: #

Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.

If there are fewer 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 leave the other as original.

Example 1: #

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

Example 2: #

Input: s = "abcd", k = 2
Output: "bacd"

Constraints: #

• 1 <= s.length <= 10⁴
• s consists of only lowercase English letters.
• 1 <= k <= 10⁴

Solution: #

public String reverseStr(String s, int k) {
  String res = "";
  int index = 0,
      flag = 0,
      len = s.length();
  while (index < len) {
    StringBuilder sb = new StringBuilder(s.substring(index, (index + k > len) ? len : index + k));
    res += (flag++ % 2 == 0) ? sb.reverse() : sb;
    index += k;
  }
  return res;
}

... #