# Question

Formatted question description: https://leetcode.ca/all/384.html

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 <= 50
• -106 <= nums[i] <= 106
• All the elements of nums are unique.
• At most 104 calls in total will be made to reset and shuffle.

# Algorithm

Traverse each position of the array, randomly generate a coordinate position each time, and then exchange the number of the current traverse position and the randomly generated coordinate position.

If the array has n numbers, then we also randomly exchange n sets of positions, thus achieving the purpose of shuffling.

It should be noted here that i + rand()% (res.size()-i) cannot be written as rand()% res.size().

# Code

• import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class Shuffle_an_Array {

// 1. array deep copy
// 2. random.nextInt to dynamic list size
class Solution {
private int[] array;
private int[] original;

private Random rand = new Random();

private List<Integer> getArrayCopy() {
List<Integer> asList = new ArrayList<Integer>();
for (int i = 0; i < array.length; i++) {
}
return asList;
}

public Solution(int[] nums) {
array = nums;
original = nums.clone();
}

public int[] reset() {
array = original;
original = original.clone();
return array;
}

public int[] shuffle() {
List<Integer> aux = getArrayCopy();

for (int i = 0; i < array.length; i++) {
int removeIdx = rand.nextInt(aux.size());
array[i] = aux.get(removeIdx);
aux.remove(removeIdx);
}

return array;
}
}
/**
* 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();
*/
}

############

class Solution {
private int[] nums;
private int[] original;
private Random rand;

public Solution(int[] nums) {
this.nums = nums;
this.original = Arrays.copyOf(nums, nums.length);
this.rand = new Random();
}

public int[] reset() {
nums = Arrays.copyOf(original, original.length);
return nums;
}

public int[] shuffle() {
for (int i = 0; i < nums.length; ++i) {
swap(i, i + rand.nextInt(nums.length - i));
}
return nums;
}

private void swap(int i, int j) {
int t = nums[i];
nums[i] = nums[j];
nums[j] = t;
}
}

/**
* 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();
*/

• // OJ: https://leetcode.com/problems/shuffle-an-array/
// Time:
//      Solution: O(N)
//      reset: O(1)
//      shuffle: O(N)
// Space: O(N)
class Solution {
private:
vector<int> nums;
public:
Solution(vector<int> nums) : nums(nums) {}

vector<int> reset() {
return nums;
}

vector<int> shuffle() {
vector<int> ans(nums);
for (int i = nums.size(); i > 1; --i) {
int j = rand() % i;
swap(ans[i - 1], ans[j]);
}
return ans;
}
};

• class Solution:
def __init__(self, nums: List[int]):
self.nums = nums
self.original = nums.copy()

def reset(self) -> List[int]:
self.nums = self.original.copy()
return self.nums

def shuffle(self) -> List[int]:
for i in range(len(self.nums)):
j = random.randrange(i, len(self.nums))
self.nums[i], self.nums[j] = self.nums[j], self.nums[i]
return self.nums

# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.reset()
# param_2 = obj.shuffle()

############

class Solution(object):

def __init__(self, nums):
"""

:type nums: List[int]
:type size: int
"""
self.nums = nums
self.reset = lambda: self.nums

def shuffle(self):
"""
Returns a random shuffling of the array.
:rtype: List[int]
"""
nums = self.nums + []
for i in reversed(range(0, len(nums))):
idx = random.randrange(0, i + 1)
nums[i], nums[idx] = nums[idx], nums[i]
return nums

# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.reset()
# param_2 = obj.shuffle()


• type Solution struct {
nums, original []int
}

func Constructor(nums []int) Solution {
return Solution{nums, append([]int(nil), nums...)}
}

func (this *Solution) Reset() []int {
copy(this.nums, this.original)
return this.nums
}

func (this *Solution) Shuffle() []int {
n := len(this.nums)
for i := range this.nums {
j := i + rand.Intn(n-i)
this.nums[i], this.nums[j] = this.nums[j], this.nums[i]
}
return this.nums
}

/**
* Your Solution object will be instantiated and called as such:
* obj := Constructor(nums);
* param_1 := obj.Reset();
* param_2 := obj.Shuffle();
*/

• class Solution {
private nums: number[];

constructor(nums: number[]) {
this.nums = nums;
}

reset(): number[] {
return this.nums;
}

shuffle(): number[] {
const n = this.nums.length;
const res = [...this.nums];
for (let i = 0; i < n; i++) {
const j = Math.floor(Math.random() * n);
[res[i], res[j]] = [res[j], res[i]];
}
return res;
}
}

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


• /**
* @param {number[]} nums
*/
const Solution = function (nums) {
this.nums = nums || [];
};

/**
* Resets the array to its original configuration and return it.
* @return {number[]}
*/
Solution.prototype.reset = function () {
return this.nums;
};

/**
* Returns a random shuffling of the array.
* @return {number[]}
*/
Solution.prototype.shuffle = function () {
let a = this.nums.slice();
for (let i = 0; i < a.length; i++) {
let rand = Math.floor(Math.random() * (a.length - i)) + i;
let tmp = a[i];
a[i] = a[rand];
a[rand] = tmp;
}
return a;
};

/**
* Your Solution object will be instantiated and called as such:
* var obj = Object.create(Solution).createNew(nums)
* var param_1 = obj.reset()
* var param_2 = obj.shuffle()
*/


• use rand::Rng;
struct Solution {
nums: Vec<i32>,
}

/**
* &self means the method takes an immutable reference.
* If you need a mutable reference, change it to &mut self instead.
*/
impl Solution {
fn new(nums: Vec<i32>) -> Self {
Self { nums }
}

fn reset(&self) -> Vec<i32> {
self.nums.clone()
}

fn shuffle(&mut self) -> Vec<i32> {
let n = self.nums.len();
let mut res = self.nums.clone();
for i in 0..n {
res.swap(i, j);
}
res
}
}

/**
* Your Solution object will be instantiated and called as such:
* let obj = Solution::new(nums);
* let ret_1: Vec<i32> = obj.reset();
* let ret_2: Vec<i32> = obj.shuffle();
*/