# 15. 3Sum

[15. 3 Sum](https://leetcode.com/problems/3sum/)

```python
# target: 0

# [-1, -1, 2, -1]
# -> [-1, -1, 2], [-1, 2, -1] choose one

# Approach 
#     For each number
#         1. target - nums[0]: 1
#         2. Find two number in the rest of array which sum to -1
#             -> Become a smaller question. it is 2 sum    

# Approach for twoSum:
#     First sort the array. 
#     We can use binary search to find the solution. 
#     [-1, -1, 0, 1, 2] target = 0
#     left: -1, right: 2 -> 1 and 1 > 0 the sum is too large
#     left: -1, right: 1 -> 0 we found the target 
    
            
class Solution:
    def twoSum(self, nums: List[int], target) -> List[List[int]]:
        left = 0
        right = len(nums) - 1
        res = []
        while left < right:
            leftNum = nums[left]
            rightNum = nums[right]
            total = leftNum + rightNum
            if total == target:
                res.append([leftNum, rightNum])
                while left < right and leftNum == nums[left]:
                    left += 1
                while left < right and rightNum == nums[right]:
                    right -= 1
            elif total < target:
                left += 1
            elif total > target:
                right -= 1
        return res
        
        
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        
        res = []
        nums.sort()
        if len(nums) <= 2:
            return []
        i = 0
        while i < len(nums):
            num = nums[i]
            candidates = self.twoSum(nums[i+1:], 0 - num)
            for candidate in candidates:
                candidate.append(num)
                res.append(candidate)
            i += 1
            while i < len(nums) and num == nums[i]:
                i += 1
                
        return res
        
```


---

# 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://garylai.gitbook.io/algorithm-and-data-structure/classic-problems/nsum/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.