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.

Example:

Input: nums = [-2,0,1,3], and target = 2
Output: 2 
Explanation: 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?

 

class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        Arrays.sort(nums);
        int res = 0;
        for (int i = 0; i < nums.length - 2; i++) {
            int start = i + 1, end = nums.length - 1;
            while (start < end) {
                if (nums[i] + nums[start] + nums[end] < target) {
                    // assume the end - 1... met 
                    res += end - start;
                    start += 1;
                } else {
                    end -= 1;
                }   
            }
        }
        return res;
    }
}