Software Engineering

Reverse a String in-place using Java

The challenge

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

Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.

You may assume all the characters consist of printable ascii characters.

Example 1:

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

Example 2:

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

The solution

The goal here is to not create a new array, or string, but to rather make the change in-place using the single character array.

class Solution {
    // the input char array
    public void reverseString(char[] s) {
        // loop through the array, but only the first half
        for (int i=0; i<s.length/2; i++) {
            // create a tmp var for first and last
            char first = s[i];
            char last = s[s.length-1-i];
            // swap the variables around
            s[i] = last;
            s[s.length-1-i] = first;