[LeetCode] 259. 3Sum Smaller

Given an array of n integers nums and an integer 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 1:

Input: nums = [-2,0,1,3], target = 2
Output: 2
Explanation: Because there are two triplets which sums are less than 2:
[-2,0,1]
[-2,0,3]

Example 2:

Input: nums = [], target = 0
Output: 0

Example 3:

Input: nums = [0], target = 0
Output: 0

Constraints:

n == nums.length
0 <= n <= 3500
-100 <= nums[i] <= 100
-100 <= target <= 100

较小的三数之和。

思路跟 3sum 差不多，但是这个题要找的是有多少种组合能满足三数之和小于 target。思路还是先固定一个数字，然后另两个数字是通过双指针逼近的方式做。

时间O(n^2)

空间O(1)

Java实现

 1 class Solution {
 2     public int threeSumSmaller(int[] nums, int target) {
 3         int res = 0;
 4         Arrays.sort(nums);
 5         for (int i = 0; i < nums.length - 2; i++) {
 6             int left = i + 1;
 7             int right = nums.length - 1;
 8             while (left < right) {
 9                 if (nums[i] + nums[left] + nums[right] < target) {
10                     res += right - left;
11                     left++;
12                 } else {
13                     right--;
14                 }
15             }
16         }
17         return res;
18     }
19 }