398. Random Pick Index

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LeetCode

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Problem

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);

Code

class Solution {
    HashMap<Integer, List<Integer>> map;
    Random rand;
    
    public Solution(int[] nums) {
        rand = new Random();
        map = new HashMap<>();

        for (int i = 0; i < nums.length; ++i) {
            if (!map.containsKey(nums[i])) {
                map.put(nums[i], new ArrayList<>());
            }
            map.get(nums[i]).add(i);
        }
    }
    
    public int pick(int target) {
        int size = map.get(target).size();
        return map.get(target).get(rand.nextInt(size));
    }
}