leetcode 16. 3Sum Closest (k-sum问题,two pointer)

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

思路: 排序,然后two pointer,复杂度 O(n^2)

 

leetcode 18. 4Sum (k-sum问题,two pointer)

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note: The solution set must not contain duplicate quadruplets.

思路: O(n^2)枚举两个元素,变成2-sum问题,总体复杂度O(n^3)

hash的解法以后补

 

leetcode 15. 3Sum (k-sum问题,two pointer)

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

 

思路:排序O(nlgn),然后枚举一个元素O(n),对于每个元素,在剩下的区间中 two pointer O(n)

整体复杂度 O(n^2)。

hash的解法以后补。