398. Random Pick Index

Problem

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 * 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.

Solution

class Solution {
    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);
 */

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).