## 259. 3Sum Smaller

Given an array of `n` integers `nums` and an integer `target`, find the number of index triplets `i`, `j`, `k` with `0 <= i < j < k < n` that satisfy the condition `nums[i] + nums[j] + nums[k] < target`.

Follow up: Could you solve it in `O(n2)` runtime?

Example 1:

```Input: nums = [-2,0,1,3], target = 2
Output: 2
Explanation: Because there are two triplets which sums are less than 2:
[-2,0,1]
[-2,0,3]
```

Example 2:

```Input: nums = [], target = 0
Output: 0
```

Example 3:

```Input: nums = [0], target = 0
Output: 0
```

Constraints:

• `n == nums.length`
• `0 <= n <= 300`
• `-100 <= nums[i] <= 100`
• `-100 <= target <= 100`

## Rust Solution

``````struct Solution;

impl Solution {
fn three_sum_smaller(mut nums: Vec<i32>, target: i32) -> i32 {
nums.sort_unstable();
let n = nums.len();
let mut res = 0;
for i in 0..n {
let mut j = i + 1;
let mut k = n - 1;
while j < k {
if nums[i] + nums[j] + nums[k] < target {
res += k - j;
j += 1;
} else {
k -= 1;
}
}
}
res as i32
}
}

#[test]
fn test() {
let nums = vec![-2, 0, 1, 3];
let target = 2;
let res = 2;
assert_eq!(Solution::three_sum_smaller(nums, target), res);
}
``````

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