Time: 1083 ms (14.78%), Space: 22.4 MB (5.02%) - LeetHub

0015-3sum/0015-3sum.py

@@ -0,0 +1,54 @@
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        # store frequency counts O(n)
+        frequencies = {}
+        for num in nums:
+            frequencies[num] = frequencies.get(num, 0) + 1
+            
+
+        # find all pairs O(n^2)
+        tripleSet = set()
+
+        def addTriple(x,y,z): # O(1) *sort always on 3 items
+            triple = [x, y, z]
+            triple.sort()
+            tripleSet.add(tuple(triple))
+
+        freq_keys = list(frequencies.keys())
+        for i in range(len(freq_keys)):
+            for j in range(i, len(freq_keys)):
+                if i == j and frequencies[freq_keys[i]] < 2:
+                    continue
+
+                pair = [freq_keys[i], freq_keys[j]]
+                needed = -(pair[0] + pair[1])
+
+                # - check for third in frequency counts O(1)
+                if frequencies.get(needed, -1) == -1:
+                    # no triple for given pair
+                    continue 
+                elif needed == pair[0] or needed == pair[1]:
+                    if pair[0] == pair[1]:
+                        if frequencies[needed] > 2:
+                            addTriple(pair[0], pair[1], needed)
+                    elif frequencies[needed] > 1:
+                        addTriple(pair[0], pair[1], needed)
+                else:
+                    addTriple(pair[0], pair[1], needed)
+                    
+
+        return list(map(lambda tup: list(tup), tripleSet))
+
+        """
+        O(n^3) *too slow
+        triples = set()
+        for i in range(len(nums)):
+            for j in range(i + 1, len(nums)):
+                for k in range(j + 1, len(nums)):
+                    if nums[i] + nums[j] + nums[k] == 0:
+                        t = [nums[i], nums[j], nums[k]]
+                        t.sort()
+                        triples.add(tuple(t))
+
+        return list(map(lambda tup: list(tup), triples))
+        """