leetcode 91. Decode Ways

Seawolf

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26
Given a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).
Example 2:

Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

dp solution

class Solution {
public:
    int numDecodings(string s) {
        
        if(s.length() == 0 || s[0] == '0') return 0;
        vector <int> dp(s.length()+1,0);
        dp[0] = 1;
        dp[1] = 1;
        int max1 = 0;
        for(int i = 2; i <= s.length(); i++){
            
            if(s[i-1] >= '1' && s[i-1] < '27'){
                dp[i] += dp[i-1];
            }
            
            if(s[i-2] == '1' || (s[i-2] == '2' && s[i-1] >= '0' && s[i-1] <= '6')){
                dp[i] += dp[i-2];
            }
            
            
        }
        return dp[s.length()];
    }
};