LeetCode: 3Sum

Question

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]
]

Explaination

For time optimization, we need to sort the given array. Then enumerate the elements from index 0(assume value v0), try to find the 2 elements which some sum of them is -v0.

Time complexity is: \[ O(N^2) \]

Code

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> ans;
        sort(nums.begin(), nums.end());

        if(nums.size() < 3) return ans;
        for(int i=0; i<nums.size(); i++) {
            int l = i+1, r = nums.size() - 1;
            int v = -nums[i];
            while(l < r) {
                int s = nums[l] + nums[r];
                if (s == v) { 
                    vector<int> e {nums[i], nums[l], nums[r]};
                    ans.push_back(e);
                    l++, r--;

                    //skip same elements
                    while(l < r && nums[l] == nums[l-1]) l++;
                    while(l < r && nums[r+1] == nums[r]) r--;
                }
                else if(s > v) r--;
                else l++;
            }
            while(i+1 < nums.size() && nums[i+1] == nums[i]) 
                i++;
        }
        return ans;
    }
};
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