Random Pick Index

题目地址：
https://leetcode.com/problems/random-pick-index/#/description

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);

解题思路：
这道题只用一个map来存就好。

代码：

Map<Integer, List<Integer>> map = new HashMap<>();
public RandomPickIndex(int[] nums) {
    for(int i = 0; i <= nums.length - 1; i++){
        int num = nums[i];
        if(map.containsKey(num)){
            List<Integer> list = map.get(num);
            list.add(i);
            map.put(num, list);
        }
        else{
            List<Integer> list = new ArrayList<>();
            list.add(i);
            map.put(num, list);
        }
    }
}
public int pick(int target) {
    List<Integer> list = map.get(target);
    Random random = new Random();
    int index = random.nextInt(list.size());
    return list.get(index);
}