398. Random Pick Index - LeetCode

Question

398. Random Pick Index

Solution

思路：重点是如果数据中有多个数target相等，要从这些数中随机取一个，根据例题

假设输入是：
int[] nums = new int[]{1, 2, 3, 3, 3};
int target = 3;

模拟：
1  != target pass
2  != target pass
3  == target pick的概率 nextInt(1)==0的概率 1,返回这个index的概念是1 - 1/3 -1/3 = 1/3
3  == target pick的概率 nextInt(2)==0的概率 1/2, 返回这个index的概率是1/2 * 2/3 = 1/3
3  == target pick的概率 nextInt(3)==0的概率 1/3, 返回这个index的概率是1/3

Java实现：

class Solution {
    private int[] nums;
    private Random random;

    public Solution(int[] nums) {
        this.nums = nums;
        random = new Random();
    }

    public int pick(int target) {
        int index = -1;
        int count = 0;
        for (int i=0; i<nums.length; i++) {
            if (nums[i] == target && random.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);
 */

Reference

• https://gitee.com/okokabcd/leetcode