README.md

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 ith 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 (Rust)

1. Solution

use std::collections::HashMap;

impl Solution {
    pub fn count_pairs(deliciousness: Vec<i32>) -> i32 {
        let mut count = HashMap::new();
        let mut ret = 0_i64;

        for i in 0..deliciousness.len() {
            count
                .entry(deliciousness[i])
                .and_modify(|c| *c += 1)
                .or_insert(1);
        }

        for (&food0, c) in count.iter() {
            for i in 0..22 {
                let food1 = (1 << i) - food0;
                if food1 < food0 {
                    ret = (ret + c * count.get(&food1).unwrap_or(&0)) % 1_000_000_007;
                } else if food1 == food0 {
                    ret = (ret + c * (c - 1) / 2) % 1_000_000_007;
                } else {
                    break;
                }
            }
        }

        ret as i32
    }
}