3Sum Smaller

Source

• leetcode: 259
• 3Sum

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?

Java

public int threeSumSmaller(int[] nums, int target) {
    int n = nums.length;
    Arrays.sort(nums);

    int count = 0;
    for (int k = 0; k < n - 2; k++) {
        int i = k + 1;
        int j = n - 1;
        while (i < j) {
            int sum = nums[k] + nums[i] + nums[j];
            if (sum >= target) {
                j--;
            } else {
                count += j - i;
                i++;
            }
        }
    }
    return count;
}