学步园

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> &num) {
        vector<vector<int> > result;

        sort(num.begin(), num.end());
        for (int i=0; i<num.size(); i++) {
                if (i && num[i] == num[i-1])
                        continue;

                int p = i+1, q = int(num.size())-1;
                while (p < q) {
                        if (num[p] + num[q] == -num[i]) {
                                vector<int> triplet;
                                triplet.push_back(num[i]);
                                triplet.push_back(num[p]);
                                triplet.push_back(num[q]);
                                result.push_back(triplet);

                                while (++p < q && num[p] == num[p-1]);
                                while (--q > p && num[q] == num[q+1]); // 此句可省略
                        }
                        else if (num[p] + num[q] < -num[i])
                                ++p;
                        else
                                --q;
                }
        }
        return result;
    }
};