1711. Count Good Meals

Description

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

 

Solutions

Solution: Hash Map

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

 

JavaScript

/**
 * @param {number[]} deliciousness
 * @return {number}
 */
const countPairs = function (deliciousness) {
  const MODULO = 10 ** 9 + 7;
  const powers = Array.from({length: 22})
    .fill('')
    .map((_, index) => 2 ** index);
  const countMap = new Map();
  let result = 0;

  for (const meal of deliciousness) {
    for (const power of powers) {
      const target = power - meal;

      if (!countMap.has(target)) continue;
      result = (result + countMap.get(target)) % MODULO;
    }
    countMap.set(meal, (countMap.get(meal) ?? 0) + 1);
  }
  return result;
};