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]

Approach #1: Direct [Accepted]

Intuition and Algorithm

We will reverse each block of 2k characters directly.

Each block starts at a multiple of 2k: for example, 0, 2k, 4k, 6k, .... One thing to be careful about is we may not reverse each block if there aren't enough characters.

To reverse a block of characters from i to j, we can swap characters in positions i++ and j--.

Complexity Analysis

  • Time Complexity: , where is the size of s. We build a helper array, plus reverse about half the characters in s.

  • Space Complexity: , the size of a.

Analysis written by: @awice.

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