package com.company;

public class DecodeWays {

/**

* I used a dp array of size n + 1 to save subproblem solutions.

* dp[0] means an empty string will have one way to decode,

* dp[1] means the way to decode a string of size 1.

* I then check one digit and two digit combination

* and save the results along the way.

* In the end, dp[n] will be the end result.

*

* @param s

* @return

*/

public int numDecodings(String s) {

if (s == null || s.length() == 0) return 0;

int n = s.length();

int[] dp = new int[n + 1];

dp[0] = 1;

dp[1] = s.charAt(0) != '0' ? 1 : 0;

for (int i = 2; i <= n; i++) {

int first = Integer.valueOf(s.substring(i - 1, i));

int second = Integer.valueOf(s.substring(i - 2, i));

if (first >= 1 && first <= 9) {

dp[i] += dp[i - 1];

}

if (second >= 10 && second <= 26) {

dp[i] += dp[i - 2];

}

}

return dp[n];

}

}