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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