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public class Solution {
public int numDecodings(String s) {
    int len = s.length();
    int mod = 1000000007;
    long [] dp = new long[len + 1];
    dp[0] = 1;
    if (s.charAt(0) == '0') return 0;
    if (s.charAt(0) == '*'){
        dp[1] = 9;
    }
    else{
        dp[1] = 1;
    }
    for (int i = 2; i<=len; i++) { // i-1 the index of current character in s. 
        if (s.charAt(i-1) == '0') {
            if (s.charAt(i-2) == '1' || s.charAt(i -2) == '2') {
                dp[i] = dp[i-2];
            }
            else if(s.charAt(i-2) == '*'){
                dp[i] = 2*dp[i-2];
            }
            else {
                return 0;
            }
        }
        else if(s.charAt(i-1) >= '1' && s.charAt(i-1) <= '9') {
            dp[i] = dp[i-1];
            if (s.charAt(i-2) == '1' || (s.charAt(i-2) == '2' && s.charAt(i-1) <= '6') ){
                dp[i] = (dp[i] + dp[i-2]) % mod;
            }
            else if (s.charAt(i-2) == '*') {
                if (s.charAt (i-1) <= '6') {
                    dp[i] = (dp[i] + dp[i-2] * 2) % mod;
                }
                else {
                    dp[i] = (dp[i] + dp[i-2]) % mod;
                }
            }
        }
        else { //s.charAt(i-1) == '*'
            dp[i] = (9*dp[i-1]) % mod;
            if ( s.charAt(i-2) == '1' ){ // 11 - 19
                dp[i] = ( dp[i] + 9 * dp[i-2] ) % mod;
            }
            else if (s.charAt(i-2) == '2') { // 21 - 26
                dp[i] = ( dp[i] + 6 * dp[i-2] ) % mod;
            }
            else if (s.charAt(i - 2) == '*') {
                dp[i] = ( dp[i] + 15 * dp[i-2] ) % mod;
            }
        }
    }
    return (int)dp[len];
}
}