398. Random Pick Index

https://leetcode.com/problems/random-pick-index/

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);

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Related problems
380-insert-delete-getrandom-o1

382-linked-list-random-node
528-random-pick-with-weight
---

	class Solution {
	Map<Integer, List<Integer>> indexMap = new HashMap<Integer, List<Integer>>();
	Random random;
	public Solution(int[] nums) {
	random = new Random();
	for (int i = 0; i < nums.length; i++) {
	indexMap.putIfAbsent(nums[i], new ArrayList<Integer>());
	indexMap.get(nums[i]).add(i);
	}
	}
	public int pick(int target) {
	List<Integer> indexes = indexMap.get(target);
	return indexes.get(random.nextInt(indexes.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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