Given an array of n integers nums and a 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.

For example, given nums = [-2, 0, 1, 3], and target = 2.

Return 2. Because there are two triplets which sums are less than 2:

[-2, 0, 1] [-2, 0, 3] Follow up: Could you solve it in O(n2) runtime?

Solution

public class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        if(nums.length < 3) return 0;
        Arrays.sort(nums);
        int cnt = 0;
        for(int i = 0; i < nums.length-2; i++) {
            int j = i+1;
            int k = nums.length-1;

            while(j < k) {
                while(j < k && nums[j] + nums[k] < target - nums[i]) j++;
                if(j != k) {
                    cnt += j - i - 1;
                    k--;
                }
            }
            // k == j
            if(k - i >= 2) cnt += (k - i) * (k - i - 1) / 2;
        }
        return cnt;
    }
}