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);

**Reservoir Sampling**

public class Solution { int[] nums; Random r; public Solution(int[] nums) { this.nums = nums; r = new Random(); } public int pick(int target) { int idx = 0; int count = 0; for (int i = 0; i < nums.length; i++) { if (nums[i] != target) continue; // for the nth target, the prob of returning is 1/n // for the (n-1)th target, the prob of returning is (n-1)/n * 1/(n-1) = 1/n if (r.nextInt(++count) == 0) { idx = i; } } return idx; } }

Advertisements