0259. 3Sum Smaller

https://leetcode.com/problems/3sum-smaller

Description

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.

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 <= 3500

• -100 <= nums[i] <= 100

• -100 <= target <= 100

ac

// n sum, two pointers, opposite direction

class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        // sort
        // walk one pass. index i, l, r, i < nums.length - 2
        // while i+l+r >= target, r--. -> add to result, r - l.
        // l++, while i+l+r >= target, r--

        // edge cases
        if (nums.length < 3) return 0;

        // init
        int res = 0;
        Arrays.sort(nums);

        // loop
        for (int i = 0; i < nums.length - 2; i++) {
            int l = i + 1, r = nums.length - 1;
            while (l < r) {
                while (l < r && nums[i] + nums[l] + nums[r] >= target) r--;
                if (l == r) break;
                res += r - l;
                l++;
            }
        }

        return res;
    }
}