Backtracking

Author: rahul22mrk

57. 17. Letter Combinations of a Phone Number.java

@@ -0,0 +1,40 @@
+class Solution {
+    HashMap<Character,String>hm;
+    List<String>ans;
+    public List<String> letterCombinations(String digits) {
+        ans=new ArrayList<>();
+        if(digits==null|| digits.length()==0){
+            return ans;
+        }
+        //initalize map and mapped the char and connected string
+        hm = new HashMap<>();
+        hm.put('2',"abc");
+        hm.put('3',"def");
+        hm.put('4',"ghi");
+        hm.put('5',"jkl");
+        hm.put('6',"mno");
+        hm.put('7',"pqrs");
+        hm.put('8',"tuv");
+        hm.put('9',"wxyz");
+        solve(digits,0,new StringBuilder());
+        return ans;
+        
+    }
+
+    private void solve(String digits,int index,StringBuilder sb){
+        if(index>=digits.length()){
+            ans.add(sb.toString());
+            return ;
+        }
+        String connectedLetters = hm.get(digits.charAt(index));
+        for(int i=0;i<connectedLetters.length();i++){
+            char ch = connectedLetters.charAt(i);
+            //add this to our temp ans -sb
+            sb.append(ch);
+            //traverse for the next char of index
+            solve(digits,index+1,sb);
+            //now remove the appended char from sb
+            sb.deleteCharAt(sb.length()-1);
+        }
+    }
+}