LeetCode-in-Java

384. Shuffle an Array

Medium

Given an integer array nums, design an algorithm to randomly shuffle the array. All permutations of the array should be equally likely as a result of the shuffling.

Implement the Solution class:

• Solution(int[] nums) Initializes the object with the integer array nums.
• int[] reset() Resets the array to its original configuration and returns it.
• int[] shuffle() Returns a random shuffling of the array.

Example 1:

Input

["Solution", "shuffle", "reset", "shuffle"] 
[[[1, 2, 3]], [], [], []]

Output:

[null, [3, 1, 2], [1, 2, 3], [1, 3, 2]]

Explanation:

Solution solution = new Solution([1, 2, 3]); 
solution.shuffle(); // Shuffle the array [1,2,3] and return its result. 
                    // Any permutation of [1,2,3] must be equally likely to be returned. 
                    // Example: return [3, 1, 2] 
solution.reset();   // Resets the array back to its original configuration [1,2,3]. Return [1, 2, 3] 
solution.shuffle(); // Returns the random shuffling of array [1,2,3]. Example: return [1, 3, 2]

Constraints:

• 1 <= nums.length <= 200
• -106 <= nums[i] <= 106
• All the elements of nums are unique.
• At most 5 * 104 calls in total will be made to reset and shuffle.

Solution

import java.util.Random;

@SuppressWarnings("java:S2245")
public class Solution {
    private int[] nums;
    private Random random;

    public Solution(int[] nums) {
        this.nums = nums;
        this.random = new Random();
    }

    // Resets the array to its original configuration and return it.
    public int[] reset() {
        return this.nums;
    }

    // Returns a random shuffling of the array.
    public int[] shuffle() {
        int[] shuffled = this.nums.clone();
        for (int i = nums.length - 1; i > 0; i--) {
            int j = random.nextInt(i + 1);
            swap(shuffled, i, j);
        }
        return shuffled;
    }

    private void swap(int[] shuffled, int i, int j) {
        int tmp = shuffled[i];
        shuffled[i] = shuffled[j];
        shuffled[j] = tmp;
    }
}

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