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?

 1 public class Solution {
 2     public int ThreeSumSmaller(int[] nums, int target) {
 3         if (nums == null || nums.Length < 3) return 0;
 4         
 5         Array.Sort(nums);
 6         
 7         int count = 0;
 
 8         
 9         for (int i = 0; i < nums.Length - 2; i++)
10         {
11             int start = i + 1, end = nums.Length - 1;
12             
13             while (start < end)
14             {
15                 if (nums[i] + nums[start] + nums[end] < target)
16                 {
17
 
                     count += end - start;
18                     start++;
19                 }
20                 else
21                 {
22                     end--;
23                 }
24             }
25         }
26         
27         return count;
28     }
29 }

Leetcode 259: 3Sum Smaller