91. Decode Ways

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

Problem

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.

The answer is guaranteed to fit in a 32-bit integer.

Example 1:

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

Example 2:

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

Example 3:

Input: s = "0"
Output: 0
Explanation: There is no character that is mapped to a number starting with '0'. We cannot ignore a zero when we face it while decoding. So, each '0' should be part of "10" --> 'J' or "20" --> 'T'.

Example 4:

Input: s = "1"
Output: 1

Constraints:

• 1 <= s.length <= 100

• s contains only digits and may contain leading zero(s).

Solution

class Solution {
public:
    int numDecodings(string s) {
        if (s[0] == '0') return 0;
        const int n = s.size();
        int dp[n + 1];
        dp[0] = dp[1] = 1;
        for (int i = 2; i <= n; ++i) {
            int dd = (s[i - 2] - '0') * 10 + (s[i - 1] - '0');
            int d = (s[i - 1] - '0');   
            if (dd == 10 || dd == 20) dp[i] = dp[i - 2];
            else if (10 < dd && dd < 27) dp[i] = dp[i - 1] + dp[i - 2];
            else if (0 < d) dp[i] = dp[i - 1];
            else return 0;
        }
        return dp[n];
    }
};

• #dp