91. Decode Ways

class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        vector<int> dp(n + 1);

        dp[0] = 1;
        dp[1] = (s[0] == '0' ? 0 : 1);

        for (int i = 2; i <= n; i++) {
            if (s[i - 1] != '0') {
                dp[i] = dp[i - 1];
            }

            int twoDigits = stoi(s.substr(i - 2, 2));

            if (10 <= twoDigits && twoDigits <= 26) {
                dp[i] += dp[i - 2];
            }
        }
        return dp[n];
    }
};

• T: $O(N)$
• S: $O(N)$