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: 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]
]

方法： 排序+三个标记，算法复杂度（O²)。

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        sort(nums.begin(),nums.end());
        vector<vector<int>> res;
        if(nums.size() < 3)
            return res;

        for(int first = 0; first < nums.size() -2; ++first){
            if(first != 0 && nums[first] == nums[first-1])
                continue;

            int second = first + 1, third = nums.size() - 1;
            while(second < third){
                while(second != first + 1 && nums[second] == nums[second - 1]&&second < third)
                    ++second;
                if(second >= third)
                    break;
                while(third != nums.size() - 1 && nums[third] == nums[third+1]&&second < third)
                    --third;
                if(second >= third)
                    break;

                int temp_sum = nums[first] + nums[second] + nums[third];
                if(temp_sum > 0)
                    --third;
                else if(temp_sum < 0)
                    ++second;
                else{
                    res.push_back(vector<int>{nums[first],nums[second],nums[third]});
                    ++second;
                    --third;
                }
            }
        }

        return res;
    }
};