花花酱 LeetCode 1711. 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: Hashtable

Same idea as LeetCode 1: Two Sum

Use a hashtable to store the occurrences of all the numbers added so far. For a new number x, check all possible 2^i – x. ans += freq[2^i – x] 0 <= i <= 21

Time complexity: O(22n)
Space complexity: O(n)