【3Sum】cpp

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:

• 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)

代码：

class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
            std::vector<std::vector<int> > ret_vector;
            if (num.size() < 3)
            {
                return ret_vector;
            }
            // sort the vector
            std::sort(num.begin(), num.end());
            // visit all the left side element of the current element
            const int target = 0;
            std::vector<int>::iterator end = num.end();
            for (std::vector<int>::iterator i = num.begin(); i != end-2; ++i){
                // ignore first duplicate i
                if ( i > num.begin() && *i==*(i-1)) continue;
                std::vector<int>::iterator j = i+1;
                std::vector<int>::iterator k = end-1;
                while (j<k){
                    const int tmp = *i+*j+*k;
                    if ( tmp < target ){
                        ++j;
                        while ( *j==*(j-1) && j<k ) ++j;
                    }
                    else if ( tmp > target ){
                        --k;
                        while ( *k==*(k+1) && j<k ) --k;
                    }
                    else{
                        int tmp_triple[3] = {*i,*j,*k};
                        std::vector<int> tmp_vector(tmp_triple,tmp_triple+3);
                        ret_vector.push_back(tmp_vector);
                        ++j;
                        --k;
                        while( *j==*(j-1) && *k==*(k+1) && j<k ) {
                            ++j;
                            --k;
                        }
                    }
                }
            }
            return ret_vector;
    }
};

Tips:

1. 先对传入的vector排序，然后从前向后遍历。原则是：包含*i元素，且满足条件的triple都加入到返回结果中

2. 这里比较困难的一点是排除重复的triple，大概想一想原则，一些极端的case只能试验出来了

另，在mac上编辑的，不知道为什么用数组初始化vector编译不通过，所以采用了比较麻烦的传参方式