398. 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);

 含义：给定一个可能包含重复元素的整数数组，给定一个目标数，随机输出其下标。你可以假设给定的目标数一定存在于数组中。

注意：数组长度可能很大。使用较多额外空间的解决方案无法通过系统测试。

class Solution {

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

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int param_1 = obj.pick(target);
 */