Leetcode 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.

Similar Questions: Decode Ways II 

思路：由后向前遍历，p[i]表示的是s[i,...,s.length()-1]中可以解码的数量。设置辅助位p[s.length()]=1。对于当前位，

1.s[i]==0，那么p[i]=0；

2.s[i]!=0，如果s[i, i+1]组成的整数不大于26，那么p[i]=p[i+1]+p[i+2]；

3.s[i]!=0，如果s[i, i+1]组成的整数大于26，那么p[i]=p[i+1];

代码：

public class Solution {
    public int numDecodings(String s) {
        if(s == null || s.length() == 0) return 0;
        int len = s.length(), i = len - 1;
        int[] p = new int[len + 1];
        p[len] = 1;
        if(s.charAt(i) != '0') p[i] = 1;
        for(i--; i >= 0; i--) {
            if(s.charAt(i) == '0') p[i] = 0;
            else p[i] = (Integer.parseInt(s.substring(i, i + 2)) <= 26) ? p[i + 2] + p[i + 1] : p[i + 1];
        }
        return p[0];
    }
}