Random Pick Index

题目
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);

答案

class Solution {
    Random r = new Random();
    int[] nums;
    public Solution(int[] nums) {
        this.nums = nums;
    }
    
    public int pick(int target) {
        int occurences = 0;
        List<Integer> list = new ArrayList<>();
        for(int i = 0; i < nums.length; i++) {
            if(nums[i] == target) {
                list.add(i);
            }
        }
        if(list.size() == 0) return -1;
        return list.get(new Random().nextInt(list.size()));
    }
}

以下是一个TLE的答案

class Solution {
    Map<Integer, List<Integer>> map;
    public Solution(int[] nums) {
        map = new HashMap<>();
        int i = 0;
        for(int num : nums) {
            List<Integer> list = map.get(num);
            if(list == null) {
                list = new ArrayList<>();
                map.put(num, list);
            }
            list.add(i++);
        }
    }
    
    public int pick(int target) {
        List<Integer> list = map.get(target);
        return list.get(new Random().nextInt(list.size()));
    }
}