Leetcode 259 3Sum Smaller

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 1: i iterate through the array, j, k is the typical two sum > or < target issue. O(n^2) solution.

 public class Solution {  
   public int threeSumSmaller(int[] nums, int target) {  
     int n=nums.length;  
     if (n<3) return 0;  
     Arrays.sort(nums);  
     int res=0;  
     for (int i=0; i<n; i++) {  
       int j=i+1, k=n-1;  
       while (j<k) {  
         int sum=nums[i]+nums[j]+nums[k];  
         if (sum<target) {  
           res+=k-j;  
           j++;  
         }  
         else k--;  
       }  
     }  
     return res;  
   }  
 }