259. 3Sum Smaller

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

public class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        if (nums == null || nums.length < 3) return 0;
        Arrays.sort(nums);
        int result = 0;

        for (int i = 0; i < nums.length - 2; i++) {
            int left = i + 1;
            int right = nums.length - 1;

            while (left < right) {
                if (nums[i] + nums[left] + nums[right] < target) {
                    result += right - left;
                    left++;
                } else {
                    right--;
                }
            }
        }

        return result;
    }
}