## [leetcode.com]算法题目 - 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.

1 class Solution {
2 public:
3     int numDecodings(string s) {
4         // Start typing your C/C++ solution below
5         // DO NOT write int main() function
6
7     }
8 };

 1 class Solution {
2 public:
3     int numDecodings(string s) {
4         // Start typing your C/C++ solution below
5         // DO NOT write int main() function
6         int len = s.length();
7
8         if (0==len || '0'==s.at(0)) return 0;
9
10         if (1==len) return 1;
11
12         if (2==len){
13             int t1 = s.at(0)-'0';
14             int t2 = s.at(1)-'0';
15
16             t2 += t1*10;
17
18             if(10 == t2 || 20 == t2)
19                 return 1;
20             else if(t2<=26)
21                 return 2;
22             else if(0==t2%10)
23                 return 0;
24             else
25                 return 1;
26         }
27
28         int *record = new int[len];
29         record[0]=numDecodings(s.substr(len-1,1));
30         record[1]=numDecodings(s.substr(len-2,2));
31
32         for(int k=2;k<len;k++){
33             string s_string = s.substr(len-k-1,k+1);
34
35             int a = s_string.at(0)-'0';
36             if (0==a)
37                 record[k]=0;
38             else if (a>2)
39                 record[k]= record[k-1];
40             else if (1==a)
41                 record[k]= record[k-1]+record[k-2];
42             else // (2==a)
43             {
44                 int kk = s_string.at(1)-'0';
45                 if(kk>6)
46                     record[k]= record[k-1];
47                 else
48                     record[k]= record[k-1]+record[k-2];
49             }
50         }
51         int result = record[len-1];
52         delete[] record;
53         return result;
54
55     }
56 };

posted on 2013-09-10 00:00  Horstxu  阅读(195)  评论(0编辑  收藏  举报