LeetCode 259. 3Sum Smaller

Question

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

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

Solution

public int threeSumSmaller(int[] nums, int target) {
    Arrays.sort(nums);
    int count = 0;
    for (int i = 0; i < nums.length - 2; i++) {
        // a variant of two-sum
        int j = i + 1;
        int k = nums.length - 1;
        while (j < k) {
            int sum = nums[i] + nums[j] + nums[k];
            if (sum < target) {
                // for current j and k,
                // all k' between j and k satisfy the requirement
                count += k - j;
                j++;
            } else {
                // if sum >= target,
                // move right, try to reduce the value of sum
                k--;
            }
        }
    }
    return count;
}