384

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;
    }
};