[leetcode]Decode Ways

问题描写叙述：

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.

基本思想：

本题利用动态规划记录前面的结果。难点是对0的处理。假设0前面是1或2以外的不论什么数字都无法解码。

代码：

public int numDecodings(String s) {  //java
        //deal with "0"
        if(s.startsWith("0"))
            return 0;

        int size = s.length();
        for(int i = 1; i< size; i++)
            if(s.charAt(i) == '0'&&(s.charAt(i-1)>'2'||s.charAt(i-1)=='0'))
                return 0;

        if(size <=1)
            return size;

        int top = -1;
        int [] record = new int [size+1];
        record[0] = 0;
        record[1] = 1;
        if(Integer.valueOf(s.substring(0,2)) <=26&&
                Integer.valueOf(s.substring(0,2))>=11&&
                Integer.valueOf(s.substring(0,2))!=20)
            record[2] = 2;
        else record[2] = 1;

        for(int i = 3; i <=size; i++){
            if(Integer.valueOf(s.substring(i-2,i))==10 ||
                Integer.valueOf(s.substring(i-2,i))==20)
                record[i] = record[i-2];
            else if(Integer.valueOf(s.substring(i-2,i))<=26&&
                Integer.valueOf(s.substring(i-2,i))>=11)
                record[i] = record[i-2]+record[i-1];
            else record[i] = record[i-1];
        }
        return record[size];
    }