_639.java

package com.fishercoder.solutions;

/**
 * 639. Decode Ways II
 *
 * A message containing letters from A-Z is being encoded to numbers using the following mapping way:

 'A' -> 1
 'B' -> 2
 ...
 'Z' -> 26
 Beyond that, now the encoded string can also contain the character '*',
 which can be treated as one of the numbers from 1 to 9.

 Given the encoded message containing digits and the character '*',
 return the total number of ways to decode it.

 Also, since the answer may be very large, you should return the output mod 109 + 7.

 Example 1:
 Input: "*"
 Output: 9
 Explanation: The encoded message can be decoded to the string: "A", "B", "C", "D", "E", "F", "G", "H", "I".
 Example 2:
 Input: "1*"
 Output: 9 + 9 = 18

 Note:
 The length of the input string will fit in range [1, 105].
 The input string will only contain the character '*' and digits '0' - '9'.

 */
public class _639 {
    /**reference: https://leetcode.com/articles/decode-ways-ii/#approach-2-dynamic-programming-accepted*/
    int m = 1000000007;

    public int numDecodings(String s) {
        long[] dp = new long[s.length() + 1];
        dp[0] = 1;
        dp[1] = s.charAt(0) == '*' ? 9 : s.charAt(0) == '0' ? 0 : 1;
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) == '*') {
                dp[i + 1] = 9 * dp[i];
                if (s.charAt(i - 1) == '1') {
                    dp[i + 1] = (dp[i + 1] + 9 * dp[i - 1]) % m;
                } else if (s.charAt(i - 1) == '2') {
                    dp[i + 1] = (dp[i + 1] + 6 * dp[i - 1]) % m;
                } else if (s.charAt(i - 1) == '*') {
                    dp[i + 1] = (dp[i + 1] + 15 * dp[i - 1]) % m;
                }
            } else {
                dp[i + 1] = s.charAt(i) != '0' ? dp[i] : 0;
                if (s.charAt(i - 1) == '1') {
                    dp[i + 1] = (dp[i + 1] + dp[i - 1]) % m;
                } else if (s.charAt(i - 1) == '2' && s.charAt(i) <= '6') {
                    dp[i + 1] = (dp[i + 1] + dp[i - 1]) % m;
                } else if (s.charAt(i - 1) == '*') {
                    dp[i + 1] = (dp[i + 1] + (s.charAt(i) <= '6' ? 2 : 1) * dp[i - 1]) % m;
                }
            }
        }
        return (int) dp[s.length()];
    }
}