15. 3Sum.cpp

/*
Given an array nums of n integers, are there elements a, b, c in nums 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.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

Sort first, then do comparison, early break, etc..
*/
class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        sort( nums.begin(), nums.end() );
        vector<vector<int>> res;
        if ( nums.empty() || nums.front() > 0 || nums.back() < 0 ) return res;
        for ( int i = 0; i <= (int)nums.size() - 3; ++i )
        {
            if ( nums[i] > 0 ) break;
            if ( i > 0 && nums[i] == nums[i - 1] ) continue;
            int target = -nums[i];
            int j = i + 1, k = nums.size() - 1;
            while ( j < k )
            {
                if ( nums[j] + nums[k] == target )
                {
                    res.push_back( {nums[i], nums[j], nums[k]} );
                    while ( j < k && nums[j + 1] == nums[j] ) ++j;
                    while ( j < k && nums[k - 1] == nums[k] ) --k;
                    ++j;
                    --k;
                }
                else if ( nums[j] + nums[k] < target )
                    ++j;
                else
                    --k;
            }
        }
        return res;
    }
};