384. Shuffle an Array

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.

Companies:
Microsoft, Apple, Google, Amazon, Snapchat

Related Topics:
Array, Math, Randomized

Solution 1.

// OJ: https://leetcode.com/problems/shuffle-an-array/
// Author: github.com/lzl124631x
// 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;
    }
};