384. Shuffle an Array

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：Reservoir Sampling

思路：
Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

class Solution {

    public Solution(int[] nums) {
        // copy a original
        copy = new int[nums.length];
        for(int i = 0; i < nums.length; i++) {
            copy[i] = nums[i];
        }
        
        // cur
        cur = new int[nums.length];
        for(int i = 0; i < nums.length; i++) {
            cur[i] = nums[i];
        }
        
        rand = new Random();
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        return copy;
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        int slow = 0;
        for(int fast = 0; fast < cur.length; fast++) {
            slow = rand.nextInt(fast + 1);
            swap(cur, fast, slow);
        }
        return cur;
    }
    
    private void swap(int[] nums, int a, int b) {
        int tmp = nums[a];
        nums[a] = nums[b];
        nums[b] = tmp;
    }
    
    private int[] copy;
    private int[] cur;
    private Random rand;
}