Solution.py

"""

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);


"""


class Solution:

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

    def pick(self, target: int) -> int:
        choice = -1
        count = 0
        for i, e in enumerate(self.nums):
            if e != target:
                continue
            if random.randint(0, count) == 0:
                choice = i
            count += 1
        return choice
        


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