leetcode@ [91] Decode Ways (Dynamic Programming)

https://leetcode.com/problems/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.

class Solution {
public:
    bool check(char a, char b) {
        int na = a - '0', nb = b - '0';
        if(na == 0) return false;
        if(na*10 + nb >= 1 && na*10 + nb <= 26) return true;
        return false;
    }
    int numDecodings(string s) {
        if(s.length() == 0) return 0;

        vector<int> dp(s.length(), 0);
        dp[0] = (s[0]=='0')? 0: 1;
        if(s.length() == 1) return dp[0];

        if(dp[0] == 0) return 0;
        else {
            bool flag = check(s[0], s[1]);
            if(s[1] == '0' && flag) dp[1] = 1;
            else if(s[1] == '0' && !flag) return 0;
            else if(s[1] != '0' && flag) dp[1] = 2;
            else if(s[1] != '0' && !flag) dp[1] = 1;
        }

        for(int i=2;i<s.length();++i) {
            if(s[i] == '0') {
                if(check(s[i-1], s[i])) dp[i] = dp[i-2];
                else return 0;
            }
            else {
                if(check(s[i-1], s[i])) dp[i] = dp[i-1] + dp[i-2];
                else dp[i] = dp[i-1];
            }
        }

        return dp[s.length()-1];
    }
};

leetcode 91: Decode Ways