[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 res = 0;
        for (int i = 0; i < nums.length - 2; i++) {
            int left = i + 1;
            int right = nums.length - 1;
            while (left < right) {
                if (nums[i] + nums[left] + nums[right] < target) {
                    res += right - left;
                    left++;
                } else {
                    right--;
                }
            }
        }
        return res;
    }
}

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