Reverse String II

Reverse String II is a Easy Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Two Pointers pattern, in the String Reversal subpattern.

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

Topics and companies

Topics: Two Pointers, String.

Reported in interviews at Apple.

How to practise Reverse String II on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(n) time and O(1) space.

Open Reverse String II in the code editor, or read the Reverse String II editorial for a worked solution with its approach and complexity analysis.

Where Reverse String II sits in the DSA pattern sheet

• Two Pointers pattern guide
• String Reversal subpattern
• The full DSA patterns sheet
• All coding practice problems

Problems related to Reverse String II

Other problems that use the same String Reversal and Two Pointers techniques:

• Reverse String — Easy, same subpattern
• Reverse Vowels of a String — Easy, same subpattern
• Reverse Words in a String — Medium, same subpattern
• Backspace String Compare — Easy, same pattern
• Boats to Save People — Medium, same pattern
• Container With Most Water — Medium, same pattern