189. Rotate Array 🔄

Difficulty: Medium - Tags: Array, Two Pointers, Mathematics

Description 📋

Given an integer array nums, rotate the array to the right by k steps, where k is non-negative.

Examples 🌟

Example 1:

Input:

nums = [1,2,3,4,5,6,7]
k = 3

Output:

[5,6,7,1,2,3,4]

Explanation:

• Rotate 1 step to the right: [7,1,2,3,4,5,6]

• Rotate 2 steps to the right: [6,7,1,2,3,4,5]

• Rotate 3 steps to the right: [5,6,7,1,2,3,4]

Example 2:

Input:

nums = [-1,-100,3,99]
k = 2

Output:

[3,99,-1,-100]

Explanation:

• Rotate 1 step to the right: [99,-1,-100,3]

• Rotate 2 steps to the right: [3,99,-1,-100]

Constraints ⚙️

• 1 <= nums.length <= 10^5

• -2^31 <= nums[i] <= 2^31 - 1

• 0 <= k <= 10^5

Solution 💡

The time complexity is $O(n)$, and the space complexity is $O(1)$ (in-place rotation). The approach uses three array reversals:

1. Reverse the entire array.

2. Reverse the first k elements.

3. Reverse the remaining n - k elements.

Java

class Solution {
    public void rotate(int[] nums, int k) {
        k = k % nums.length; // In case k > nums.length
        reverse(nums, 0, nums.length - 1);   // Step 1: Reverse the entire array
        reverse(nums, 0, k - 1);             // Step 2: Reverse the first k elements
        reverse(nums, k, nums.length - 1);   // Step 3: Reverse the rest of the array
    }
    
    private void reverse(int[] nums, int start, int end) {
        while (start < end) {
            int temp = nums[start];
            nums[start] = nums[end];
            nums[end] = temp;
            start++;
            end--;
        }
    }
}

You can find the full Solution1.java file here and Solution2.java file here. Bad solution need to avoid here