[LeetCode] 384. Shuffle an Array

Problem

Shuffle a set of numbers without duplicates.

Example:

// Init an array with set 1, 2, and 3.
int[] nums = {1,2,3};
Solution solution = new Solution(nums);

// Shuffle the array [1,2,3] and return its result. Any permutation of [1,2,3] must equally likely to be returned.
solution.shuffle();

// Resets the array back to its original configuration [1,2,3].
solution.reset();

// Returns the random shuffling of array [1,2,3].
solution.shuffle();

Solution

Note

1. Random random = new Random();
2. manipulate on a copy of given array, so that reset() would work;
3. int j = random.nextInt(i+1); then swap A[i] and A[j].

class Solution {

    private int[] N;
    private Random random;
    public Solution(int[] nums) {
        this.N = nums;
        random = new Random();
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        return N;
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        if (N == null) return null;
        int[] A = N.clone();
        for (int i = 0; i < A.length; i++) {
            int j = random.nextInt(i+1);
            swap(A, i, j);
        }
        return A;
    }
    
    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}