3Sum Smaller.java

/*
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?

-2 0 1 3
i  l r r 
r - l = 3 - 1 = 2 ways

-2 0 1 3
i    l r
only one way

time = O(n^2) + O(nlogn) = O(n^2)
space = O(1)
*/

class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        if (nums == null || nums.length < 3) {
            return 0;
        }
        Arrays.sort(nums);
        int count = 0;
        for (int i = 0; i < nums.length - 2; i++) {
            int left = i + 1;
            int right = nums.length - 1;
            while (left < right) {
                int sum = nums[i] + nums[left] + nums[right];
                if (sum < target) {
                    count += right - left;
                    left++;
                } else {
                    right--;
                }
            }
        }
        return count;
    }
}