Added leetcode exercise 17

leetcode/17_letter_combinations_of_a_phone_number.py

@@ -0,0 +1,21 @@
+class Solution:
+    def letterCombinations(self, digits: str) -> List[str]:
+        if len(digits) == 0:
+            return []
+
+        options = {
+            '2': ('a', 'b', 'c'),
+            '3': ('d', 'e', 'f'),
+            '4': ('g', 'h', 'i'),
+            '5': ('j', 'k', 'l'),
+            '6': ('m', 'n', 'o'),
+            '7': ('p', 'q', 'r', 's'),
+            '8': ('t', 'u', 'v'),
+            '9': ('w', 'x', 'y', 'z')
+        }
+
+        pools = (options[digit] for digit in digits)
+        result = [""]
+        for pool in pools:
+            result = [x+y for x in result for y in pool]
+        return result