52. Letter Combinations of a Phone Number

Author: ahmlvs

17. Letter Combinations of a Phone Number/solution.py

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+# Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
+
+# A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
+
+
+ 
+
+# Example 1:
+
+# Input: digits = "23"
+# Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+# Example 2:
+
+# Input: digits = ""
+# Output: []
+# Example 3:
+
+# Input: digits = "2"
+# Output: ["a","b","c"]
+ 
+
+# Constraints:
+
+# 0 <= digits.length <= 4
+# digits[i] is a digit in the range ['2', '9'].
+
+from typing import List
+
+class Solution:
+    def letterCombinations(self, digits: str) -> List[str]:
+        if not digits:
+            return []
+
+        phone_map = {
+            "2": "abc",
+            "3": "def",
+            "4": "ghi",
+            "5": "jkl",
+            "6": "mno",
+            "7": "pqrs",
+            "8": "tuv",
+            "9": "wxyz"
+        }
+        result = []
+
+        # for "23"
+        # index=0 -> 2 -> path=["a"] + backtrack for next index
+        # then path=["b"] + backtrack for next index
+
+        def backtrack(index, path):
+            # if index == len(digits) need to stop
+            if index == len(digits):
+                result.append("".join(path))
+                return
+            
+            possible_letters = phone_map[digits[index]]
+            for letter in possible_letters:
+                # add letter to path
+                # and call backtrack next index
+                path.append(letter)
+                backtrack(index+1, path)
+                # delete last letter from path
+                path.pop()
+        
+        # start from 0 index of digits
+        # and nothing in path
+        backtrack(0, [])
+        return result

17. Letter Combinations of a Phone Number/test_solution.py

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+import unittest
+from solution import Solution
+
+
+class TestSolution(unittest.TestCase):
+    def test_1(self):
+        digits = "23"
+        expected = ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+        self.assertCountEqual(Solution().letterCombinations(digits), expected)
+    
+    def test_2(self):
+        digits = ""
+        expected = []
+        self.assertCountEqual(Solution().letterCombinations(digits), expected)
+    
+    def test_3(self):
+        digits = "2"
+        expected = ["a","b","c"]
+        self.assertCountEqual(Solution().letterCombinations(digits), expected)