Random Pick Index Problem


Description

LeetCode Problem 398.

Given an integer array nums 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.

Implement the Solution class:

  • Solution(int[] nums) Initializes the object with the array nums.
  • int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i’s, then each index should have an equal probability of returning.

Example 1:

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Input
["Solution", "pick", "pick", "pick"]
[[[1, 2, 3, 3, 3]], [3], [1], [3]]
Output
[null, 4, 0, 2]
Explanation
Solution solution = new Solution([1, 2, 3, 3, 3]);
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(1); // It should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.

Constraints:

  • 1 <= nums.length <= 2 * 10^4
  • -2^31 <= nums[i] <= 2^31 - 1
  • target is an integer from nums.
  • At most 10^4 calls will be made to pick.


Sample C++ Code

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class Solution {
    unordered_map<int, vector<int>> map;
public:
    Solution(vector<int>& nums) {
        for(int i=0;i<nums.size();i++){
            map[nums[i]].push_back(i);
        }
    }
    
    int pick(int target) {
        vector<int> random = map[target];
        return random[rand()%random.size()];
    }
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(nums);
 * int param_1 = obj->pick(target);
 */




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