Is Reverse String II easy or hard?

Reverse String II is classified as an Easy problem. The challenge lies in carefully handling index boundaries and partial segments when fewer than k or 2k characters remain. Once the 2k iteration pattern is understood, the implementation is straightforward.

Reverse String II Python/Java solution

In Python, convert the string to a list of characters and reverse segments using index swaps. In Java, convert the string to a char array and apply the same two-pointer logic. Both implementations follow the same O(n) time algorithm with segment jumps of 2k.

How to solve Reverse String II in O(n)?

Process the string in intervals of 2k. For each starting index i, reverse the substring from i to min(i + k - 1, n - 1) using two pointers that swap characters inward. Then jump to i + 2k and repeat until the string ends.

What is the best approach for Reverse String II?

The most efficient approach is the in-place two-pointer method. Iterate through the string in steps of 2k and reverse only the first k characters of each block. This runs in O(n) time and uses O(1) extra space since swaps happen directly on a character array.

Is Reverse String II asked at Google/Amazon/Meta?

String manipulation and two-pointer problems like Reverse String II frequently appear in coding interviews at companies such as Amazon, Google, and Meta. While the exact question may vary, the pattern of reversing segments and handling edge cases is common in interview rounds.

What data structure is used in Reverse String II?

The main structure used is a mutable character array derived from the input string. The algorithm then applies the two-pointer technique to swap characters within the required range. Some implementations also use a string builder when constructing a new result string.

What is the time complexity of Reverse String II?

The time complexity is O(n), where n is the length of the string. Each character is visited at most once while reversing segments of length k in every 2k block. No nested loops scale with n, so the runtime remains linear.

Description Solution Video 5:34

541. Reverse String II

Easy54.7% acceptance7 companies

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

Asked at 7 companies

Google Meta Bloomberg

Infosys Accenture Amazon Microsoft

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String Two Pointers

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541. Reverse String II

Asked at 7 companies

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