15. 3Sum

题目

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.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
[-1, 0, 1],
[-1, -1, 2]
]

思路

本题，先给数组排序，固定一个数，寻找另外两个与其和为0的数，中间使用set<vector<int>> 临时存储数组，这样避免重复值的出现

代码

class Solution {public:    vector<vector<int>> threeSum(vector<int>& nums) {        vector<vector<int>> res;        set<vector<int>> result;        if(nums.size()<3)            return res;        sort(nums.begin(),nums.end());        for(int i=0;i<nums.size()-2;i++)        {            int l = i+1,r = nums.size()-1;            while(l<r)            {                if(nums[i]+nums[l]+nums[r]==0)                {                    vector<int> temp;                    temp.push_back(nums[i]);                    temp.push_back(nums[l]);                    temp.push_back(nums[r]);                    l++;                    r--;                    result.insert(temp);                }                else if(nums[i]+nums[l]+nums[r]<0)                    l++;                else if(nums[i]+nums[l]+nums[r]>0)                    r--;            }        }        set<vector<int>>::iterator itr = result.begin();        for(;itr!=result.end();itr++)        {            res.push_back(*itr);        }        return res;      }};