Reverse String
Easy · Backtracking · Two Pointers · String · Recursion
Problem
You are given a string as a list of characters, s. Reverse the order of the characters in place: change the list itself using only O(1) extra memory, and return nothing.
The expected output shows the contents of s after your function runs.
Examples
Input: s = ["c","o","d","e"] Output: ["e","d","o","c"]
The first and last characters swap, then the middle two.
Input: s = ["a","b","a","c"] Output: ["c","a","b","a"]
Read backwards, the list is c, a, b, a.
Input: s = ["l","o","t","u","s"] Output: ["s","u","t","o","l"]
l and s swap, o and u swap, and the middle t stays where it is.
Constraints
- 1 <= len(s) <= 10^5
- s[i] is a printable ASCII character.
Approaches
- Baseline: move the last character forward, one by one — time O(n²), space O(1)
Follow-up
What if only some characters may move, such as letters, while digits and punctuation stay put? The same two pointers work if each one skips the characters that stay. Both use cases below ask a version of that.
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