動機

要找時間看Reservoir Sampling

Problem

Given an integer array nums 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.

Implement the Solution class:

    t
  • Solution(int[] nums) Initializes the object with the array nums.
    • t
  • int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i's, then each index should have an equal probability of returning.

 

Example 1:

Input["Solution", "pick", "pick", "pick"][[[1, 2, 3, 3, 3]], [3], [1], [3]]Output[Algorithm, Leetcode, 4, 0, 2]ExplanationSolution solution = new Solution([1, 2, 3, 3, 3]);solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.solution.pick(1); // It should return 0. Since in the array only nums[0] is equal to 1.solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.

 

Constraints:

    t
  • 1 <= nums.length <= 2 * 104
    • t
  • -231 <= nums[i] <= 231 - 1
    • t
  • target is an integer from nums.
    • t
  • At most 104 calls will be made to pick.

Sol

class Solution:

    def __init__(self, nums: List[int]):
        self.ns = defaultdict(list)
        
        for (i,n) in enumerate(nums):
            self.ns[n].append(i)

    def pick(self, target: int) -> int:
        return self.ns[target][randint(0,len(self.ns[target])-1)]