[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

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。

复杂度

时间O(n^2)
空间O(1)

代码

Java实现

class Solution {
    public int threeSumSmaller(int[] nums, int target) {
        Arrays.sort(nums);
        int n = nums.length;
        int res = 0;
        for (int k = n - 1; k >= 2; k--) {
            int lo = 0;
            int hi = k - 1;
            while (lo < hi) {
                int sum = nums[lo] + nums[hi] + nums[k];
                if (sum < target) {
                    res += hi - lo;
                    lo++;
                } else {
                    hi--;
                }
            }
        }
        return res;
    }
}

相关题目

259. 3Sum Smaller
611. Valid Triangle Number
2563. Count the Number of Fair Pairs
2824. Count Pairs Whose Sum is Less than Target