【leetcode刷题笔记】Decode Ways

‘A‘ -> 1
‘B‘ -> 2
...
‘Z‘ -> 26

 1 public class Solution {
 2     private boolean isValidNum(String s,int i){
 3         if(i <= 0)
 4             return false;
 5         int first = s.charAt(i-1)-‘0‘;
 6         int second = s.charAt(i)-‘0‘;
 7         int num = first*10 + second;
 8         if(num >= 10 && num <= 26)
 9             return true;
10         return false;
11     }
12     public int numDecodings(String s) {
13         if(s==null || s.length() == 0)
14             return 0;
15         int length = s.length();
16         int[] dp = new int[length+1];
17         
18         dp[0] = 1;
19         dp[1] = s.charAt(0) == ‘0‘?0:1;
20         
21         for(int i=2;i<=length;i++){
22             if(s.charAt(i-1) != ‘0‘)
23                 dp[i]=dp[i-1];
24             
25             if(isValidNum(s, i-1))
26                 dp[i] += dp[i-2];
27         }
28         return dp[length];        
29     }
30 }