## 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`

item of food, return ^{th}*the number of different good meals you can make from this list modulo*

`10`^{9} + 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:4Explanation: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:15Explanation:The good meals are (1,1) with 3 ways, (1,3) with 9 ways, and (1,7) with 3 ways.

**Constraints:**

`1 <= deliciousness.length <= 10`

^{5}`0 <= deliciousness[i] <= 2`

^{20}

## 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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