Leetcode 91: Decode Ways

Posted 2020-10-14 Keep walking

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 public class Solution {
 2     public int NumDecodings(string s) {
 3         if (s.Length == 0 || s[0] == ‘0‘) return 0;
 4         
 5         int dp0 = 1, dp1 = 1;
 6         
 7         for (int i = 1; i < s.Length; i++)
 8         {
 9             var dp2 = 0;
10             var decode = ((int)s[i - 1] - (int)‘0‘) * 10 + ((int)s[i] - (int)‘0‘);
11             if (decode >= 10 && decode <= 26)
12             {
13                 dp2 = dp0;
14             } 
15             
16             if (decode % 10 != 0)
17             {
18                 dp2 += dp1;
19             }
20             
21             dp0 = dp1;
22             dp1 = dp2;
23         }
24         
25         return dp1;
26     }
27 }