Leetcode--3Sum

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Problem Description:

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)

分析：題目要求把所有可能的集合都找出來，因此想到的就是首先將數組排序，然後利用兩重迴圈依次選出兩個數字a和b，然後在剩下的數字中尋找是否存在c，具體實現用到了upper_bound函數找到比當前數大的第一個數，去掉重複迴圈的情況，然後用find函數尋找c是否存在，存在則將三個數記錄下來。具體代碼如下：

class Solution {public:    vector<vector<int> > threeSum(vector<int> &num) {        vector<vector<int> > results;        if(num.size()<3) return results;        vector<int> subset;        sort(num.begin(),num.end());        vector<int>::iterator p=num.begin(),q,flag;        while(p<(num.end()-2))        {            q=p+1;            while(q<num.end()-1)            {                int tag=0-*p-*q;                if(find(q+1,num.end(),tag)!=num.end())                {                flag=find(q+1,num.end(),tag);                subset.push_back(*p);                subset.push_back(*q);                subset.push_back(*flag);                results.push_back(subset);                }                subset.clear();                q=upper_bound(q,num.end()-1,*q);            }            p=upper_bound(p,num.end()-2,*p);        }        return results;    }};