number of ways to decode a string?

Question

I am working on problem where I need to decode a string..

A message containing letters from AZ is being encoded to numbers using the following mapping:

'A' -> 1

'B' -> 2

...

'Z' -> 26

Given a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"

Output: 2

Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: "226"

Output: 3

Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

I came up with below recursion approach but it is giving wrong output for this input "227". Output should be "2" but my program is giving "3":

  public static int decodeWays(String data) {
    return helper(data, data.length());
  }

  private static int helper(String data, int k) {
    if (k == 0)
      return 1;
    int s = data.length() - k;
    if (data.charAt(s) == '0')
      return 0;

    int result = helper(data, k - 1);
    if (k >= 2 && Integer.parseInt(data.substring(0, 2)) <= 26) {
      result += helper(data, k - 2);
    }
    return result;
  }

What is wrong with my above approach?

Answer 1

In this line-

if (k >= 2 && Integer.parseInt(data.substring(0, 2)) <= 26) {

You always check the same 2-digit number data.substring(0, 2) . Instead consider something like

data.substring(data.length()-k, data.length()).substring(0, 2)

or

data.substring(data.length()-k, data.length()-k+2)