0384. Shuffle an Array

https://leetcode.com/problems/shuffle-an-array

Description

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.

ac

random.nextInt() is the key here.

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

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

    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        if (nums == null) return null;
        int[] res = nums.clone();
        for (int i = 0; i < res.length; i++) {
            int k = random.nextInt(res.length);
            swap(res, i, k);
        }
        return res;
    }
    private void swap(int[] nums, int a, int b) {
        int tmp = nums[a];
        nums[a] = nums[b];
        nums[b] = tmp;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int[] param_1 = obj.reset();
 * int[] param_2 = obj.shuffle();
 */