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-# algorithms
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+# Arrays
+
++ [Three sum (Generator version, O(n^3))](#three-sum-generator-version-on3)
++ [Three sum (Set + hash version O(n^2))](#three-sum-set--hash-version-on2)
+
+## Three sum (Generator version, O(n^3))
+
+https://leetcode.com/problems/3sum/
+
+```python
+class Solution(object):
+
+    def twoSum(self, nums, target, removed):
+        """
+        :type nums: List[int]
+        :type target: int
+        :type removed: int
+        :rtype: List[int]
+        """
+
+        for index, value in enumerate(nums):
+            if index == removed:
+                continue
+            if target - value in nums and index != nums.index(target - value) and removed != nums.index(target - value):
+                yield sorted([value, target - value])
+
+    def threeSum(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: List[List[int]]
+        """
+
+        triplets = []
+        for index, value in enumerate(nums):
+            pairs = []
+            for x in self.twoSum(nums, -value, index):
+                if x not in pairs:
+                    pairs.append(x)
+            for p in pairs:
+                if sorted(p + [value]) not in triplets:
+                    triplets += [sorted(p + [value])]
+        return triplets
+
+```
+
+## Three sum (Set + hash version O(n^2))
+
+https://leetcode.com/problems/3sum/
+
+```python
+class Solution(object):
+
+    def twoSum(self, nums, target, removed):
+        """
+        :type nums: List[int]
+        :type target: int
+        :type removed: set
+        :rtype: List[int]
+        """
+
+        dict = {}
+        for index, value in enumerate(nums):
+            if target - value in dict:
+                removed.add((value, target - value, -target))
+            dict[value] = index
+        del dict
+
+    def threeSum(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: List[List[int]]
+        """
+
+        nums.sort()
+        out = set()
+        for index, value in enumerate(nums):
+            self.twoSum(nums[index + 1:], -value, out)
+        return [list(x) for x in out]
+
+```
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