【Leetcode】Decode Ways

题目链接：https://leetcode.com/problems/decode-ways/

题目：

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：

类似于爬楼梯题目。

算法：

public int numDecodings(String s) {  
    if (s.length() == 0)  
        return 0;  
    char sc[] = s.toCharArray();  
    if (sc[0] == '0')  
        return 0;  
    int c[] = new int[s.length()];  
    c[0] = 1;  
  
    for (int i = 1; i < sc.length; i++) {  
  
        if ((sc[i - 1] == '2' && sc[i] >= '1' && sc[i] <= '6') || (sc[i - 1] == '1' && sc[i] != '0')) {  
            if (i + 1 < sc.length && sc[i + 1] == '0') {  
                c[i] = c[i - 1];  
            } else if (i - 2 >= 0) {  
                c[i] = c[i - 1] + c[i - 2];// 类似爬楼梯题  
            } else {  
                c[i] = c[i - 1] + 1;  
            }  
        } else if (sc[i] == '0' && sc[i - 1] == '0') {  
            return 0;  
        } else if (sc[i] == '0' && sc[i - 1] > '2') {  
            return 0;  
        } else {  
            c[i] = c[i - 1];  
        }  
    }  
    return c[s.length() - 1];  
}