Leetcode 398. Random Pick Index

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);

Accepted

70,723

Submissions

133,199

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并不是很喜欢这题的表述，这题的意思应该是说不要对输入nums进行任何操作，或者说nums是个stream，这样用蓄水池算法理所当然，否则每次都遍历一遍有点牵强

import random
class Solution:

    def __init__(self, nums):
        self.nums = nums
        
    def pick(self, target):
        cnt,res = 0,0
        for i,num in enumerate(self.nums):
            if (num == target):
                cnt += 1
                if (cnt == 1):
                    res = i
                else:
                    rdint = random.randint(1, cnt)
                    if (rdint == 1):
                        res = i
        return res


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