add: PhoneKeypadCombinations

Author: Jer3myYu

java/Backtracking/PhoneKeypadCombinations.java

@@ -0,0 +1,42 @@
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class PhoneKeypadCombinations {
+    public List<Stirng> phoneKeypadCombinations(String digits) {
+        Map<Character, String> keypadMap = new HashMap<>();
+        keypadMap.put('2', "abc");
+        keypadMap.put('3', "def");
+        keypadMap.put('4', "ghi");
+        keypadMap.put('5', "jkl");
+        keypadMap.put('6', "mno");
+        keypadMap.put('7', "pqrs");
+        keypadMap.put('8', "tuv");
+        keypadMap.put('9', "wxyz");
+        List<String> result = new ArrayList<>();
+        backtrack(0, new ArrayList<>(), digits, keypadMap, result);
+        return result;
+    }
+
+    private void backtrack(int i, List<Character> currCombination, String digits, Map<Character, String> keypadMap, List<String> result) {
+        // Termination condition: if all digits have been considered, add the
+        // current combination to the output list.
+        if (currCombination.size() == digits.length()) {
+            StringBuilder sb = new StringBuilder();
+            for (char c: currCombination) {
+                sb.append(c);
+            }
+            result.add(sb.toString());
+            return;
+        }
+        for (char letter: keypadMap.get(digits.charAt(i)).toCharArray()) {
+            // Add the current letter.
+            currCombination.add(letter);
+            // Recursively explore all paths that branch from this combination.
+            backtrack(i + 1, currCombination, digits, keypadMap, result);
+            // Backtrack by removing the letter we just added.
+            currCombination.remove(currCombination.size() - 1);
+        }
+    }
+}