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中等
水塘抽样
哈希表
数学
随机化

398. 随机数索引

English Version

题目描述

给你一个可能含有 重复元素 的整数数组 nums ，请你随机输出给定的目标数字 target 的索引。你可以假设给定的数字一定存在于数组中。

实现 Solution 类：

  • Solution(int[] nums) 用数组 nums 初始化对象。
  • int pick(int target) 从 nums 中选出一个满足 nums[i] == target 的随机索引 i 。如果存在多个有效的索引，则每个索引的返回概率应当相等。

 

示例：

输入
["Solution", "pick", "pick", "pick"]
[[[1, 2, 3, 3, 3]], [3], [1], [3]]
输出
[null, 4, 0, 2]

解释
Solution solution = new Solution([1, 2, 3, 3, 3]);
solution.pick(3); // 随机返回索引 2, 3 或者 4 之一。每个索引的返回概率应该相等。
solution.pick(1); // 返回 0 。因为只有 nums[0] 等于 1 。
solution.pick(3); // 随机返回索引 2, 3 或者 4 之一。每个索引的返回概率应该相等。

 

提示：

  • 1 <= nums.length <= 2 * 104
  • -231 <= nums[i] <= 231 - 1
  • target 是 nums 中的一个整数
  • 最多调用 pick 函数 104 次
 

解法

方法一

Python3

class Solution:
    def __init__(self, nums: List[int]):
        self.nums = nums

    def pick(self, target: int) -> int:
        n = ans = 0
        for i, v in enumerate(self.nums):
            if v == target:
                n += 1
                x = random.randint(1, n)
                if x == n:
                    ans = i
        return ans


# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.pick(target)

Java

class Solution {
    private int[] nums;
    private Random random = new Random();

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

    public int pick(int target) {
        int n = 0, ans = 0;
        for (int i = 0; i < nums.length; ++i) {
            if (nums[i] == target) {
                ++n;
                int x = 1 + random.nextInt(n);
                if (x == n) {
                    ans = i;
                }
            }
        }
        return ans;
    }
}

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

C++

class Solution {
public:
    vector<int> nums;

    Solution(vector<int>& nums) {
        this->nums = nums;
    }

    int pick(int target) {
        int n = 0, ans = 0;
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[i] == target) {
                ++n;
                int x = 1 + rand() % n;
                if (n == x) ans = i;
            }
        }
        return ans;
    }
};

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

Go

type Solution struct {
	nums []int
}

func Constructor(nums []int) Solution {
	return Solution{nums}
}

func (this *Solution) Pick(target int) int {
	n, ans := 0, 0
	for i, v := range this.nums {
		if v == target {
			n++
			x := 1 + rand.Intn(n)
			if n == x {
				ans = i
			}
		}
	}
	return ans
}

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