https://leetcode.com/problems/random-pick-index/description/

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.

Note:
The array size can be very large. Solution that uses too much extra space will not pass the judge.

Example:

int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);

// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);

// pick(1) should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(1);

Solution: Reservoir sampling

https://en.wikipedia.org/wiki/Reservoir_sampling

Time complexity: O(query * n)

Space complexity: O(1)

C++

// Author: Huahua
// Running time: 92 ms
class Solution {
public:
  Solution(vector<int> nums) {
    nums_ = std::move(nums);
  }

  int pick(int target) {
    int n = 0;    
    int index = -1;
    for (int i = 0; i < nums_.size(); ++i) {
      if (nums_[i] != target) continue;
      ++n;
      if (rand() % n == 0) index = i;
    }
    return index;
  }
private:
  vector<int> nums_;
};

The post 花花酱 LeetCode 398. Random Pick Index appeared first on Huahua's Tech Road.

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();

C++

// Author: Huahua
// Running time: 249 ms
class Solution {
public:
  Solution(vector<int> nums) {
    nums_ = std::move(nums);
  }

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

  /** Returns a random shuffling of the array. */
  vector<int> shuffle() {
    vector<int> output(nums_);
    const int n = nums_.size();
    for (int i = 0; i < n - 1; ++i) {      
      int j = rand() % (n - i) + i;
      std::swap(output[i], output[j]);
    }
    return output;
  }
private:
  vector<int> nums_;
};

The post 花花酱 LeetCode 384. Shuffle an Array appeared first on Huahua's Tech Road.