398. Random Pick Index-优快云博客

本文介绍了一种在含重复元素的整数数组中随机选取指定目标值索引的方法。利用水塘抽样算法实现了一个类`Solution`，该类接受一个可能包含重复元素的整数数组，并能在接收到目标值时随机返回该值的一个索引位置。文章通过实例展示了如何使用这个类。

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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);

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

本题用的是水塘抽樣，这是一种随即算法，且nextInt表示第一个元素一定会被取到，而且被取到的概率是1/n，故而选==0是一定会被抽中的