15. 3Sum

• Difficulty: Medium

• Topics: Array, Two Pointers

• Similar Questions:

  • Two Sum

  • 3Sum Closest

  • 4Sum

  • 3Sum Smaller

Problem:

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]
]

Solutions:

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

        for (int i = 0; i < nums.size(); ++i) {
            if (i > 0 && nums[i] == nums[i-1]) continue; // deduplication
            int sum = -nums[i];
            int left = i + 1;
            int right = nums.size() - 1;

            while (left < right) {
                if (nums[left] + nums[right] == sum) {
                    if (result.size() == 0 || result.back()[1] != nums[left] || result.back()[2] != nums[right]) // deduplication
                        result.push_back({nums[i], nums[left], nums[right]});
                    left++;
                    right--;
                } else if (nums[left] + nums[right] > sum) {
                    right--;
                } else {
                    left++;
                }
            }
        }

        return result;
    }
};