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.

 

用DP算法来解比较简单，需维护一个num[s.length()+1]数组，用来存储前i位子串的不同方式的编码数，最后返回num[s.length()]即可

public class Solution {
    public int numDecodings(String s) {
        if(s.length()==0||s.contains("00")||(s.length()>0&&s.charAt(0)=='0')) return 0;
        int[] num = new int[s.length()+1];
        num[0] = 1;
        num[1] = 1;
        for(int i=1;i<s.length();i++) {
            int tmp = Integer.valueOf(s.substring(i-1,i+1));
            if(s.charAt(i)=='0') {
                if(tmp>=30) return 0;
                else {
                    num[i] = num[i-1];
                    num[i+1] = num[i];
                }
            }
            else {
                if(tmp>26||tmp<10) num[i+1] = num[i];
                else num[i+1] = num[i]+num[i-1];
            }
        }
        return num[s.length()];
    }
}