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17-letter-combinations-of-a-phone-number/17-letter-combinations-of-a-phone-number.py

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+class Solution(object):
+    def letterCombinations(self, digits):
+        """
+        :type digits: str
+        :rtype: List[str]
+        """
+        numToLetters = {
+            2: "abc",
+            3: "def",
+            4: "ghi",
+            5: "jkl",
+            6: "mno",
+            7: "pqrs",
+            8: "tuv",
+            9: "wxyz"
+        }
+        
+        combos = [""]
+        for d in digits:
+            comsToAdd = []
+            for l in numToLetters[int(d)]:
+                for combo in combos:
+                    comsToAdd.append(combo + l)
+            combos = comsToAdd
+        
+        return [] if combos == [""] else combos
+