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`

1711. Count Good Meals
``````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);
}
``````