Count Good Meals

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:

from collections import Counter

class Solution:
    def countPairs(self, deliciousness: List[int]) -> int:
        deliciousness = Counter(deliciousness)
        ctr = 0
        for i in range(22):
            total = 1 << i
            for d in deliciousness:
                if total - d in deliciousness:
                    if 2 * d != total:
                        ctr += deliciousness[d] * deliciousness[total - d] / 2
                    else:
                        ctr += deliciousness[d] * (deliciousness[d] - 1) // 2
        return int(ctr) % (10 ** 9 + 7)