Leetcode Problem 639. Decode Ways II

639. Decode Ways II

Leetcode Solutions

Initialize a dp array of size n + 1, where n is the length of the input string s. Set dp[0] to 1 as there is one way to decode an empty string.
Iterate over the string from the first character to the last.
For each character at index i, check if it is a *. If so, multiply dp[i] by 9 as there are 9 possible decodings for *.
Check if the current character can form a valid two-digit number with the previous character. If the previous character is 1, or 2 and the current character is between 0 and 6, or if the previous character is *, update dp[i] accordingly.
If the current character is 0, ensure it forms a valid two-digit number with the previous character; otherwise, it cannot be decoded.
Return dp[n] modulo 10^9 + 7 as the final result.

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