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🟩 Reverse String (#344)

🔗 Problem Link​

📋 Problem Statement​

Write a function that reverses a string. The input string is given as an array of characters s.

You must do this by modifying the input array in-place with O(1) extra memory.

💡 Examples​

Example 1​

Input: s = ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]

Example 2​

Input: s = ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]

🔑 Key Insights & Approach​

Core Observation: To reverse in-place, we need to swap elements from the outside moving inward. The first element swaps with the last, second with second-to-last, and so on.

Why Two Pointers?

  • In-place requirement: Can't create a new array (O(1) space constraint)
  • Symmetry: Reversing is inherently symmetric—work from both ends
  • Efficiency: Each element is touched once - O(n) time

When to Stop?

  • When left >= right (pointers meet or cross)
  • For odd-length arrays: middle element stays in place
  • For even-length arrays: all elements get swapped

Pattern Recognition: This is the classic "Two Pointers - Opposite Ends" pattern, used for reversing, palindrome checking, and container with most water. Key trait: pointers move toward each other from both ends

🐍 Solution: Python​

Approach 1: Two Pointers​

Time Complexity: O(n) | Space Complexity: O(1)

def reverse_string(s):
    left, right = 0, len(s) - 1
    while left < right:
        s[left], s[right] = s[right], s[left]
        left += 1
        right -= 1