398 Random Pick Index.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);
Author: Rajeev Ranjan

"""

import random

class Solution(object):
    def __init__(self, nums):
        """
        :type nums: List[int]
        """
        self.A = nums

    def pick(self, target):
        """
        O(n)
        Reservoir Sampling
        :type target: int
        :rtype: int
        """
        sz = 0
        ret = None
        for idx, val in enumerate(self.A):
            if val == target:
                sz += 1
                p = random.randrange(0, sz)
                if p == 0:
                    ret = idx

        return ret


class SolutionError(object):
    def __init__(self, nums):
        """
        Reservoir Sampling
        Assume pick is only called once
        :type nums: List[int]
        """
        self.d = {}
        for idx, val in enumerate(nums):
            if val not in self.d:
                self.d[val] = (idx, 1)
            else:
                prev, sz = self.d[val]
                p = random.randrange(0, sz)
                if p < sz:
                    self.d[val] = (idx, sz + 1)
                else:
                    self.d[val] = (prev, sz + 1)

    def pick(self, target):
        """
        :type target: int
        :rtype: int
        """
        return self.d[target][0]