398. Random Pick Index

We want to randomly pick an index for a number thats in a list of numbers.

To do this, we can populate a dict of num → list and then choice on that list. This solution does that.

class Solution:
    def __init__(self, nums: List[int]):
        m = defaultdict(list)
        for i, n in enumerate(nums):
            m[n].append(i)
        self.nums = m
 
    def pick(self, target: int) -> int:
        return choice(self.nums[target])