# LeetCode 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: []
```

**Constraints:**

* `0 <= nums.length <= 3000`
* `-10^5 <= nums[i] <= 10^5`

## Solution:

[English Version in Youtube](https://youtu.be/3Szdf-_uRRE)

[中文版解答Youtube Link](https://youtu.be/v86W0cgvO5Y)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1cy4y1J71A/)

```
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 - 1] == nums[i]) {
                continue;
            }
            
            int j = i + 1;
            int k = nums.size() - 1;
            
            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];
                if (sum < 0) {
                    j++;
                } else if (sum > 0) {
                    k--;
                } else {
                    result.push_back({nums[i], nums[j], nums[k]});
                    while ((j + 1) < nums.size() && nums[j] == nums[j+1]) j++;
                    j++;
                    while ((k - 1) >= 0 && nums[k] == nums[k - 1]) k--;
                    k--;
                }
            }
        }
        
        return result;
    }
};
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://zhenchaogan.gitbook.io/leetcode-solution/leetcode-15-3sum.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.