Leetcode 91. Decode Ways

Decode Ways :

給定一個寫碼字串，解碼它並返回它的解碼方法數。

Python

class Solution: 
    def numDecodings(self, s: str) -> int:
        n = len(s)
        dp = [0] * (n+1)
        dp[n] = 1
        for i in range(n-1, -1, -1):
            if s[i] != '0':  # Single digit
                dp[i] = dp[i+1]
            if i+1 < n and (s[i] == '1' or s[i] == '2' and s[i+1] <= '6'):  # Two digits
                dp[i] += dp[i+2]
        return dp[0]

C++

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