15. 3Sum

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.

Notice that the solution set must not contain duplicate triplets.

Example 1:

Input: nums = [-1,0,1,2,-1,-4]
Output: [[-1,-1,2],[-1,0,1]]

Example 2:

Input: nums = []
Output: []

Example 3:

Input: nums = [0]
Output: []

Solution

C++

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int> > res;
        int n = nums.size();
        if(n < 3) return res;
        sort(nums.begin(), nums.end());
        for(int i = 0; i <= n - 2; ++i) {
            if(i > 0 && nums[i] == nums[i-1]) continue;
            int j = i+1, k = n-1;
            while(j < k) {
                if(nums[j] + nums[k] < -nums[i]) ++j;
                else if(nums[j] + nums[k] > -nums[i]) --k;
                else {
                    res.push_back({nums[i],nums[j],nums[k]});
                    while(j < k && nums[j] == nums[j+1]) ++j;
                    while(j < k && nums[k] == nums[k-1]) --k;
                    ++j;
                    --k;
                }
            }
        }
        return res;
    }
};

Explanation