398 Random Pick Index 随机数索引
Description:
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);
题目描述:
给定一个可能含有重复元素的整数数组，要求随机输出给定的数字的索引。 您可以假设给定的数字一定存在于数组中。
注意:
数组大小可能非常大。 使用太多额外空间的解决方案将不会通过测试。
示例 :
int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);
// pick(3) 应该返回索引 2,3 或者 4。每个索引的返回概率应该相等。
solution.pick(3);
// pick(1) 应该返回 0。因为只有nums[0]等于1。
solution.pick(1);
思路:
蓄水池取样
遍历数组, 每次查找到 target, 记录出现的次数
选择一个 0-出现次数的随机数, 如果选到 0, 就交换结果和数组下标
时间复杂度O(n), 空间复杂度O(1)
代码:
C++:
class Solution
{
private:
vector<int> arr;
public:
Solution(vector<int>& nums)
{
arr = nums;
}
int pick(int target)
{
int count = 0, result = 0, n = arr.size();
for (int i = 0; i < n; i++)
{
if (arr[i] == target)
{
++count;
if (!(rand() % count)) result = i;
}
}
return result;
}
};
/**
* Your Solution object will be instantiated and called as such:
* Solution* obj = new Solution(nums);
* int param_1 = obj->pick(target);
*/
Java:
class Solution {
private int[] nums;
private Random random;
public Solution(int[] nums) {
this.nums = nums;
this.random = new Random();
}
public int pick(int target) {
int point = 0, result = 0, n = nums.length;
for (int i = 0; i < n; i++) {
if (nums[i] == target) {
++point;
if (random.nextInt(point) % point == 0) result = i;
}
}
return result;
}
}
/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(nums);
* int param_1 = obj.pick(target);
*/
Python:
class Solution:
def __init__(self, nums: List[int]):
self.nums = nums
def pick(self, target: int) -> int:
return random.choice([i for i in range(len(self.nums)) if self.nums[i] == target])
# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.pick(target)
