259 3Sum Smaller

259. 3Sum Smaller

1. Question

Given an array ofnintegersnumsand atarget, find the number of index tripletsi, j, kwith0 <= i < j < k < nthat satisfy the conditionnums[i] + nums[j] + nums[k] < target.

For example, givennums=[-2, 0, 1, 3], andtarget= 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 inO(n^2) runtime?

2. Implementation

(1) Two Pointers

class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        int res = 0;
        Arrays.sort(nums);

        for (int i = 0; i < nums.length - 2; i++) {
            int j = i + 1, k = nums.length - 1;

            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];

                if (sum < target) {
                    res += k - j;
                    ++j;
                }
                else {
                    --k;
                }
            }
        }
        return res;
    }
}

3. Time & Space Complexity

时间复杂度O(n^2), 空间复杂度O(1)