数据结构与算法[LeetCode]—3Sum 求数组中和为0 的三个数的组合

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:

Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
The solution set must not contain duplicate triplets.

For example, given array S = {-1 0 1 2 -1 -4},

A solution set is:
(-1, 0, 1)
(-1, -1, 2)

思路：直接分别进行三次遍历复杂度为）O(n^3)。

            如果我们进行排序，那么最后一个值采用二分搜索那么可以降低最后一个数查找的复杂度。总体复杂度为O(n^2logn)

#include<iostream>
#include<algorithm>
#include<vector>

using namespace std;

//先排序，后依次两两组合遍历
class Solution{
public:
vector<vector<int> > threeSum(vector<int>& num){
vector<vector<int> > result;
if(num.size()<3)return result;
sort(num.begin(),num.end());
const int target=0;

vector<int>::iterator last=num.end();
for(vector<int>::iterator a=num.begin();a<(last-2);a=upper_bound(a,last-2,*a)){
for(vector<int>::iterator b=a+1;b<(last-1);b=upper_bound(b,last-1,*b)){
int c=target-*b-*a;

if(binary_search(b+1,last,c)){
vector<int> vec;
vec.push_back(*a);
vec.push_back(*b);
vec.push_back(c);
result.push_back(vec);
}
}

}
return result;
}

};