LeetCode-3Sum Smaller

Posted 2020-08-09 LiBlog

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.length<3) return 0;
        Arrays.sort(nums);
        
        int count = 0;
        for (int p1=0;p1<nums.length-2;p1++){
            // Early termination
            if (nums[p1]+nums[p1+1]+nums[p1+2]>=target) break;
            
            int p2=p1+1, p3=nums.length-1;
            while (p2<p3){
                while (p2<p3 && nums[p2]+nums[p3]>=target-nums[p1]){
                    p3--;
                }
                count += p3-p2;
                p2++;
            }
        }
        return count;
    }
}