1995. Count Special Quadruplets

Given a 0-indexed integer array `nums`, return the number of distinct quadruplets `(a, b, c, d)` such that:

• `nums[a] + nums[b] + nums[c] == nums[d]`, and
• `a < b < c < d`

Example 1:

```Input: nums = [1,2,3,6]
Output: 1
Explanation: The only quadruplet that satisfies the requirement is (0, 1, 2, 3) because 1 + 2 + 3 == 6.
```

Example 2:

```Input: nums = [3,3,6,4,5]
Output: 0
Explanation: There are no such quadruplets in [3,3,6,4,5].
```

Example 3:

```Input: nums = [1,1,1,3,5]
Output: 4
Explanation: The 4 quadruplets that satisfy the requirement are:
- (0, 1, 2, 3): 1 + 1 + 1 == 3
- (0, 1, 3, 4): 1 + 1 + 3 == 5
- (0, 2, 3, 4): 1 + 1 + 3 == 5
- (1, 2, 3, 4): 1 + 1 + 3 == 5
```

Constraints:

• `4 <= nums.length <= 50`
• `1 <= nums[i] <= 100`

1995. Count Special Quadruplets
``````struct Solution;

impl Solution {
fn count_quadruplets(nums: Vec<i32>) -> i32 {
let n = nums.len();
let mut res = 0;
for i in 0..n {
for j in i + 1..n {
for k in j + 1..n {
for l in k + 1..n {
if nums[i] + nums[j] + nums[k] == nums[l] {
res += 1;
}
}
}
}
}
res
}
}

#[test]
fn test() {
let nums = vec![1, 2, 3, 6];
let res = 1;
assert_eq!(Solution::count_quadruplets(nums), res);
let nums = vec![3, 3, 6, 4, 5];
let res = 0;
assert_eq!(Solution::count_quadruplets(nums), res);
let nums = vec![1, 1, 1, 3, 5];
let res = 4;
assert_eq!(Solution::count_quadruplets(nums), res);
}
``````