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3Sum LeetCode Solution

·2 mins
leetcode
Mayukh Datta
Author
Mayukh Datta
Table of Contents

Given an array nums of n integers, are there elements abc 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: []

Constraints:

  • 0 <= nums.length <= 3000
  • -105 <= nums[i] <= 105

Link: https://leetcode.com/problems/3sum/

Solution #

class Solution { public: vector<vector> threeSum(vector& nums) {
int n = nums.size(); vector<vector> res;

    // we've seen in two sum II problem that sorted array
    // makes it easier to solve the problem
    sort(nums.begin(), nums.end());
    
    for(int i=0;i<n;i++) {
        
        // to skip duplicates in the sorted array
        if(i > 0 && nums\[i\] == nums\[i-1\]) {
            continue;
        }
        
        // We have to solve a+b+c = 0
        // We get the value of 'a' from i
        // Using the two pointer approach that we used in
        // two sum II, we'll try to find 'b' and 'c'
        int start = i + 1, end = n - 1, sum;
        
        while(start < end) {
            sum = nums\[i\] + nums\[start\] + nums\[end\];
            // sum = a + b + c
            
            if(sum > 0) {
                end--;
            }else if(sum < 0) {
                start++;
            }else {
                res.push\_back(vector<int>{nums\[i\], nums\[start\], nums\[end\]});
                
                // to skip duplicates present in between the start and end pointers
                start++;
                while(nums\[start\] == nums\[start - 1\] && start < end) {
                    start++;
                }
            }
        }
    }
    return res;
}

};