Add files via upload

Author: 17tanya

Recursion/LetterCombinationsOfAPhoneNumber.java

@@ -0,0 +1,44 @@
+//LeetCode 17. Letter Combinations of a Phone Number
+//Question - https://leetcode.com/problems/letter-combinations-of-a-phone-number/
+
+class Solution {
+    List<String> res;
+    Map<Character, List<Character>> map;
+    
+    public List<String> letterCombinations(String digits) {
+        int n = digits.length();
+        res = new ArrayList<>();
+        
+        if(n == 0) return res;
+        char str[] = new char[n];
+        
+        map = new HashMap<>();
+        map.put('2', Arrays.asList('a', 'b', 'c'));
+        map.put('3', Arrays.asList('d', 'e', 'f'));
+        map.put('4', Arrays.asList('g', 'h', 'i'));
+        map.put('5', Arrays.asList('j', 'k', 'l'));
+        map.put('6', Arrays.asList('m', 'n', 'o'));
+        map.put('7', Arrays.asList('p', 'q', 'r', 's'));
+        map.put('8', Arrays.asList('t', 'u', 'v'));
+        map.put('9', Arrays.asList('w', 'x', 'y', 'z'));
+        
+        helper(str, digits, 0);
+        return res;
+        
+    }
+    
+    public void helper(char str[], String digits, int index){
+        if(index == digits.length()){
+            res.add(String.valueOf(str));
+            return;
+        }
+        
+        char currChar = digits.charAt(index);
+        List<Character> chars = map.get(currChar);
+        
+        for(char c : chars){
+            str[index] = c;
+            helper(str, digits, index + 1);
+        }
+    }
+}