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

Rust Solution

struct Solution;

use std::collections::HashMap;

const MOD: i64 = 1_000_000_007;

impl Solution {
    fn count_pairs(deliciousness: Vec<i32>) -> i32 {
        let mut hm: HashMap<i32, i64> = HashMap::new();
        let mut res = 0;
        for x in deliciousness {
            for i in 0..22 {
                let sum = 1 << i;
                let y = sum - x;
                if let Some(k) = hm.get(&y) {
                    res += k;
                    res %= MOD;
                }
            }
            *hm.entry(x).or_default() += 1;
        }
        res as i32
    }
}

#[test]
fn test() {
    let deliciousness = vec![1, 3, 5, 7, 9];
    let res = 4;
    assert_eq!(Solution::count_pairs(deliciousness), res);
    let deliciousness = vec![1, 1, 1, 3, 3, 3, 7];
    let res = 15;
    assert_eq!(Solution::count_pairs(deliciousness), res);
    let deliciousness = vec![
        149, 107, 1, 63, 0, 1, 6867, 1325, 5611, 2581, 39, 89, 46, 18, 12, 20, 22, 234,
    ];
    let res = 12;
    assert_eq!(Solution::count_pairs(deliciousness), res);
}

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