Reverse Letters Then Special Characters in a String - Video Solutions

Problem Overview: Given a string containing letters and special characters, reorder it so the letters appear in reverse order and the special characters also appear in reverse order, while each group keeps its own positions relative to the other group. The task is mostly about scanning the string carefully and swapping characters that belong to the same category.

Approach 1: Simulation with Extra Storage (O(n) time, O(n) space)

The straightforward approach separates the problem into two collections. First iterate through the string and push all letters into one list and all special characters into another. Reverse both lists. Then iterate through the original string again and rebuild the result: whenever the current position contains a letter, take the next element from the reversed letter list; otherwise take from the reversed special-character list. This works because each character category is processed independently while the string traversal keeps their placement pattern intact.

Approach 2: Two Pointers Simulation (O(n) time, O(1) space)

A more space‑efficient solution uses two pointers. First collect indices of letters and reverse them in-place by moving pointers from both ends of the letter index list and swapping characters in the original string. Repeat the same logic for special characters. Each swap operation exchanges characters that belong to the same category, ensuring letters only swap with letters and special characters only swap with special characters. Because each character is visited a constant number of times, the algorithm runs in linear time.

This pattern is common in two pointers problems where you selectively swap elements that satisfy a condition. It also fits the idea of simulation: you mimic the exact transformation step by step rather than deriving a complex formula. The core operations are simple string traversal, classification using character checks, and controlled swapping.

Both implementations rely heavily on basic string manipulation. The key insight is to treat letters and special characters as two independent sequences embedded inside the same string. Once you isolate those sequences logically, reversing them becomes a standard operation.

Recommended for interviews: The two‑pointer simulation approach is typically expected. It achieves O(n) time with O(1) extra space and demonstrates that you can manipulate strings efficiently in-place. Showing the extra‑array simulation first can help explain the idea, but the in‑place two‑pointer version signals stronger problem‑solving ability.