> For the complete documentation index, see [llms.txt](https://frost.gitbook.io/iosbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://frost.gitbook.io/iosbook/leet-code/leetcode-solutions/15.-3sum.md).

# \[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.

**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]
]
```

**Analyse:**

**Solution:**

```swift
class Solution {
    
    func threeSum(_ nums: [Int]) -> [[Int]] {
        var output: [[Int]] = [];
        var sortedNums = nums.sorted{$0 < $1}
        var i = 0
         
        while (i < sortedNums.count - 2) {
            let value = sortedNums[i]
            var left = i + 1
            var right = sortedNums.count - 1
            
            while(left < sortedNums.count && right > 0 && left < right) {
                let sum = value + sortedNums[left] + sortedNums[right]
                if (sum == 0) {
                    output.append([sortedNums[left], sortedNums[right], value])
                    
                    repeat {
                        left += 1 
                    } while(left < right && sortedNums[left] == sortedNums[left - 1])
                      
                    repeat {
                        right -= 1 
                    } while(left < right && sortedNums[right] == sortedNums[right + 1])

                } else if(sum < 0) {
                  left += 1
                } else {
                  right -= 1
                }
            }
            
            repeat {
              i += 1
            }
            while((i < (sortedNums.count - 2)) && (sortedNums[i] == sortedNums[i - 1]))
        }
        
        return output 
    } 
}
```