91. Decode Ways

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

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example, Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).

The number of ways decoding "12" is 2.

DP[i] means how many ways you can decode s.substring(0, i);

public class Solution {
    public int numDecodings(String s) {
        if(s == null || s.length() == 0) return 0;
        int[] dp = new int[s.length() +1];
        //dp[i] means how many ways you can decode s.substring(0..i)
        dp[0] = 1;
        dp[1] = s.charAt(0) == '0' ? 0 : 1;
        for(int i= 2; i<= s.length(); i++){
            int tmp = Integer.parseInt(s.substring(i-1, i));
            if( tmp != 0) dp[i] = dp[i-1];// if tmp == 0, this line won't execute, but the [i-2,i-1] build a number 10, or 20,
            //the result is set to dp[i-2] directly in the next step.
            tmp = (s.charAt(i-2) - '0') * 10 + s.charAt(i-1) - '0';
            if(tmp >=10 && tmp <= 26) dp[i] += dp[i-2];
        }

        return dp[s.length()];
    }

}