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)

class Solution {public:    vector<vector<int>> threeSum(vector<int>& nums) {        sort(nums.begin(),nums.end());        vector<vector<int>> ret;        for(int i = 0;i < nums.size();i++)        {            int start =  i+1;            int end = nums.size()-1;            if(i>0&&nums[i]==nums[i-1])//关键点1：去除已经重复的点                continue;            while(start<end)//转化为2sum问题            {                while(start>i+1&&nums[start]==nums[start-1])//关键点2：排除重复的部分                {                    start++;                }                while(end<nums.size()-1&&nums[end]==nums[end+1])                {                    end--;                }                int sum = nums[start]+nums[i]+nums[end];                if(start<end)                {                    if(sum == 0)                    {                        vector<int> temp;                        temp.push_back(nums[i]);                        temp.push_back(nums[start]);                        temp.push_back(nums[end]);                        ret.push_back(temp);                        start++;                    }else if(sum>0)                    {                        end--;                    }else{                        start++;                    }                }             }        }        return ret;    }};