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);

offline algorithm

public class Solution {
    int[] data;
    Random r = new Random();
    public Solution(int[] nums) {
        data = nums;
    }

    public int pick(int target) {
        int count = 0; int res = -1;
        ArrayList<Integer> al = new ArrayList<>();
        for(int i=0; i< data.length; i++){
            if(data[i] != target) continue;
            count++;
            al.add(i);
        }
        int k = r.nextInt(count);
        return al.get(k);
    }
}

online

public class Solution {
    int[] data;
    Random r = new Random();
    public Solution(int[] nums) {
        data = nums;
    }

    public int pick(int target) {
        int count = 0; int res = -1;
        for(int i=0; i< data.length; i++){
            if(data[i] != target) continue;
            count++;
            if(r.nextInt(count) == 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);
 */