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398 Random Pick Index

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);

Solutions

class Solution {
    private int[] nums;
    public Solution(int[] nums) {
        this.nums = nums;
    }

    public int pick(int target) {
        Random r = new Random();
        int res = -1;
        int size = 0;
        for (int i = 0; i < nums.length; i ++) {
            if (nums[i] == target) {
                size ++;
                int rand = r.nextInt(size);
                if (rand == 0) {
                    res = i;
                }
            }
        }
        return res;
    }
}

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