LeetCode [398] Random Pick Index

398. Random Pick Index

Medium

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);

class Solution {
    int n;
    Random random;
    int nums[];
    public Solution(int[] nums) {
        this.n = nums.length;
        this.random = new Random();
        this.nums = nums;
    }
    
    /*
    ret = 2 : 1*(1/2)*(2/3) = 1/3
    ret = 3 : (1/2)*(2/3) = 1/3
    ret = 4 : (1/3)
    */
    public int pick(int target) {
        int ret = -1;
        int count = 0;
        for(int i=0; i<this.n; ++i){
            if(this.nums[i]!=target) continue;
            if(this.random.nextInt(++count) == 0){
                ret = i;
            }
        }
        return ret;
    }
}

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