[leetcode] 【数组】15. 3Sum

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)

题解

题意和two sum类似。方法也是那三种，a遍历3遍；b排序再夹逼；c遍历先取一个元素，剩下两个用two sum的标记法。
注意要采取去重措施。
这里我用的是方法b，先排序，再夹逼。

//cppclass Solution {public:    vector<vector<int>> threeSum(vector<int>& nums) {        vector<vector<int> > res;        if(nums.size()<3)            return res;        sort(nums.begin(),nums.end());        auto tail=nums.end();        int target=0;        for(auto first=nums.begin();first<nums.end()-2;first++)        {            if(*first==*(first-1)&&first!=nums.begin())                continue;            auto second=first+1;            auto third=tail-1;            while(second<third)            {                if(*first+*second+*third<target)                    second++;                else if(*first+*second+*third>target)                    third--;                else                 {                    res.push_back({*first,*second,*third});                    second++;                    third--;                }            }        }        sort(res.begin(),res.end());        res.erase(unique(res.begin(),res.end()),res.end());        return res;    }};