15.3Sum

3Sum

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Question Solution 

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)

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class Solution {

public:
vector<vector<int> > threeSum(vector<int> &num) {
int N = num.size();
vector<vector<int>> res;
if (N<3) return res;//判错
sort(num.begin(), num.end());//数组排序


//遍历第一个数，剩下两个数用双指针判断
for (int i = 0; i<N - 2; ++i){
int left = i + 1, right = N - 1;
if (num[i]>0) break;//优化
while (left<right){
if (num[i] + num[left]>0 || num[right] < 0) break;//优化
if ((num[i] + num[left] + num[right]) == 0){
res.push_back(vector<int>({ num[i], num[left], num[right] }));
//去重
while(left<right&&num[left]==num[left+1]){
       ++left;
   }
   ++left;
                while(i<N&&num[i]==num[i+1]){
                    ++i;
                }
}else if (num[i] + num[left] + num[right]>0){
                --right;
}else{
   ++left;
}
}
}
return res;
}
};