20. Shuffle an Array.cpp

/*
Shuffle an Array
================

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.

Hint #1  
The solution expects that we always use the original array to shuffle() else some of the test cases fail. 
*/

class Solution
{
public:
  vector<int> original;

  Solution(vector<int> &nums)
  {
    original = nums;
  }

  /** Resets the array to its original configuration and return it. */
  vector<int> reset()
  {
    return original;
  }

  /** Returns a random shuffling of the array. */
  vector<int> shuffle()
  {
    unordered_set<int> st;
    vector<int> ans;
    int N = original.size();

    while (st.size() != N)
    {
      int val = rand() % N;
      if (!st.count(val))
      {
        st.insert(val);
        ans.push_back(original[val]);
      }
    }
    return ans;
  }
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(nums);
 * vector<int> param_1 = obj->reset();
 * vector<int> param_2 = obj->shuffle();
 */