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:

  1. [-2, 0, 1]
  2. [-2, 0, 3]

Follow up:

Could you solve it in O(n^2) runtime?

Solution:

  1. public class Solution {
  2.   public int threeSumSmaller(int[] nums, int target) {
  3.     Arrays.sort(nums);
  5.     int n = nums.length, count = 0;
  7.     for (int i = 0; i < n - 2; i++) {
  8.       int j = i + 1, k = n - 1;
  10.       while (j < k) {
  11.         int sum = nums[i] + nums[j] + nums[k];
  13.         if (sum >= target) {
  14.           k--;
  15.         } else {
  16.           count += k - j;
  17.           j++;
  18.         }
  19.       }
  20.     }
  22.     return count;
  23.   }
  24. }

Time complexity: O(n^2)