398. Random Pick Index - jiejackyzhang/leetcode-note GitHub Wiki

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);

解题思路为reservoir sampling。

target在nums中有n个，则每个被选中的概率为1/n。

public class Solution {
    
    private int[] nums;
    private Random r;

    public Solution(int[] nums) {
        this.nums = nums;
        r = new Random();
    }
    
    public int pick(int target) {
        int index = -1;
        int count = 1;
        for(int i = 0; i < nums.length; i++) {
            if(nums[i] == target && (r.nextInt(count++) == 0)) {
                index = i;
            }
        }
        return index;
    }
}

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