LeetCode-in-Java

398. Random Pick Index

Medium

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:

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 * 104
• -231 <= nums[i] <= 231 - 1
• target is an integer from nums.
• At most 104 calls will be made to pick.

Solution

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Random;

@SuppressWarnings("java:S2245")
public class Solution {
    // O(n) time | O(n) space
    private Map<Integer, List<Integer>> map;
    private Random rand;

    public Solution(int[] nums) {
        map = new HashMap<>();
        rand = new Random();
        for (int i = 0; i < nums.length; i++) {
            map.computeIfAbsent(nums[i], k -> new ArrayList<>()).add(i);
        }
    }

    public int pick(int target) {
        List<Integer> list = map.get(target);
        return list.get(rand.nextInt(list.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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