# 17. Letter Combinations|Time:O(3^N*4^M)|Space: O(3^N*4^M)

a guest
Aug 19th, 2019
84
Never
Not a member of Pastebin yet? Sign Up, it unlocks many cool features!
1. // Problem: https://leetcode.com/problems/letter-combinations-of-a-phone-number
2. // Solution: https://leetcode.com/problems/letter-combinations-of-a-phone-number/solution/
3.
4. class Solution {
5.   Map<String, String> phone = new HashMap<String, String>() {{
6.     put("2", "abc");
7.     put("3", "def");
8.     put("4", "ghi");
9.     put("5", "jkl");
10.     put("6", "mno");
11.     put("7", "pqrs");
12.     put("8", "tuv");
13.     put("9", "wxyz");
14.   }};
15.
16.   List<String> output = new ArrayList<String>();
17.
18.   public void backtrack(String combination, String next_digits) {
19.     // if there is no more digits to check
20.     if (next_digits.length() == 0) {
21.       // the combination is done
23.     }
24.     // if there are still digits to check
25.     else {
26.       // iterate over all letters which map
27.       // the next available digit
28.       String digit = next_digits.substring(0, 1);
29.       String letters = phone.get(digit);
30.       for (int i = 0; i < letters.length(); i++) {
31.         String letter = phone.get(digit).substring(i, i + 1);
32.         // append the current letter to the combination
33.         // and proceed to the next digits
34.         backtrack(combination + letter, next_digits.substring(1));
35.       }
36.     }
37.   }
38.
39.   public List<String> letterCombinations(String digits) {
40.     if (digits.length() != 0)
41.       backtrack("", digits);
42.     return output;
43.   }
44. }
RAW Paste Data