LeetCode 91. Decode Ways

题目如下：

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.

思路：

思路很简单，就是要检查s.charAt(i)和s.char(i-1)是否能构成一个字母，如果不能res[i]=res[i-1],如果能res[i]＝res[i-1]＋res[i-2];

但难处在于处理s.charAt(i)=='0'的各种情况，还有边界条件要处理好。

本体代码：

/**
 * Created by yuanxu on 17/4/13.
 */
public class DP91 {

    public static int numDecodings(String s) {
        int len = s.length();
        if (len == 0) return 0;
        if (s.charAt(0) == '0')  return 0;

        // for both s.char(i) and res[i], make i begins with 1
        s = '0' + s;
        int res[] = new int[len+1];

        res[0] = 1; // for convince
        res[1] = 1;

        for (int i=2; i<=len; i++) {
            if (s.charAt(i) == '0') {
                if (s.charAt(i-1) =='1' || s.charAt(i-1) =='2') {
                    res[i] = res[i-2];
                } else {
                    return 0;
                }
            } else {
                res[i] = res[i-1];
                if (s.charAt(i-1) == '1'|| (s.charAt(i-1)=='2' && s.charAt(i) < '7')) {
                    res[i] += res[i-2];
                }
            }
        }
        return res[len];
    }

    /**
     *
     * @test
     */
    public  static void main(String[] args) {
//        String s = "12312";
//        String s = "0";
//        String s = "01";
//        String s = "10";
        String s = "110";
        System.out.println(numDecodings(s));
    }
}