91. Decode Ways

https://leetcode.com/problems/decode-ways/description/

class Solution {
public:
    int numDecodings(string s) {
        int n = s.length();
        if (n == 0)     return 0;
        vector<int> dp(n, 0);
        for (int i = 0; i < n; i++) {
            if (s[i] > '0' && s[i] <= '9')
                dp[i] += i == 0 ? 1 : dp[i-1];
            if (i > 0 && (s[i-1] == '1' || s[i-1] == '2' && s[i] <= '6'))
                dp[i] += i == 1 ? 1 : dp[i-2];
        }
        return dp[n-1];
    }
};