Coding

Reverse an array

Asked at

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

Given an array arr of n elements. The task is to reverse the given array. The reversal of array should be inplace.

Input: n=5, arr = [1,2,3,4,5]

Output: [5,4,3,2,1]

Explanation: The reverse of the array [1,2,3,4,5] is [5,4,3,2,1]

Input: n=6, arr = [1,2,1,1,5,1]

Output: [1,5,1,1,2,1]

Explanation: The reverse of the array [1,2,1,1,5,1] is [1,5,1,1,2,1].

Input: n=3, arr = [1,2,1]

1 <= n <= 104 1 <= arr[i] <= 105

cpp

class Solution{
public:
    void reverse(int arr[], int n){
        
    }
};

java

class Solution {
    public void reverse(int[] arr, int n) {
   
    }
}

python

class Solution:
    def reverse(self, arr: list, n: int) -> None:

javascript

class Solution {
    // Function to reverse the array in place
    // The reversal should be done IN-PLACE.
    // Do not return anything from this function.
    reverse(arr, n) {
        // Your code here
    }
}

csharp

public class Solution
{
    public void reverse(int[] arr, int n)
    {
   
    }
}

func reverse(arr []int, n int) {

}

Stuck? Show a way to structure it+

01Set left to zero and right to n-1.
02Swap the values at both pointers.
03Move left forward and right backward.
04Stop when pointers meet or cross.

Reference answer

Reverse in place with two pointers. Start left=0 and right=n-1; while left<right, swap the values, then increment left and decrement right. Every swap puts two elements into their final positions, so time is O(n) and extra space is O(1). The strict comparison avoids reswapping the center element, though swapping it would be harmless. The loop invariant is that all positions before left and after right already hold their final reversed values; the unknown region shrinks by two positions per swap. That makes correctness immediate at termination. Handle empty and single-element arrays naturally: the condition is false and no mutation occurs. Do not allocate a reversed copy when the contract requires mutation in place, and do not try to swap the values by arithmetic because that risks overflow.

Then expect these follow-ups

How would you reverse a linked list?
Tests: pointer manipulation
How would you reverse only a subarray?
Tests: index boundaries

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