Leetcode - Count Good Meals Solution

A good meal is a meal that contains exactly two different food items with a sum of deliciousness equal to a power of two.

You can pick any two different foods to make a good meal.

Given an array of integers deliciousness where deliciousness[i] is the deliciousness of the i​​​​​​th​​​​​​​​ item of food, return the number of different good meals you can make from this list modulo 109 + 7.

Note that items with different indices are considered different even if they have the same deliciousness value.

Example 1:

Input: deliciousness = [1,3,5,7,9]
Output: 4
Explanation: The good meals are (1,3), (1,7), (3,5) and, (7,9).
Their respective sums are 4, 8, 8, and 16, all of which are powers of 2.

Example 2:

Input: deliciousness = [1,1,1,3,3,3,7]
Output: 15
Explanation: The good meals are (1,1) with 3 ways, (1,3) with 9 ways, and (1,7) with 3 ways.

Constraints:

• 1 <= deliciousness.length <= 105
• 0 <= deliciousness[i] <= 220

Solution in Python

class Solution:
    def countPairs(self, deliciousness: List[int]) -> int:
        def sumOfAP(n):
            return n*(n-1)//2
        c = Counter(deliciousness)
        powerOfTwos = {2**i for i in range(22)}
        possibilities = 0
        for x in c.keys():
            if x*2 in powerOfTwos :
                possibilities += sumOfAP(c[x])
            for i in powerOfTwos:
                y = i - x
                if y != x and y>-1:
                    possibilities += c[x]*c[y]
            c[x] = 0
        return possibilities%(10**9 + 7)