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_639.java
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_639.java
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/**
* LeetCode 639 - Decode Ways II
* <p>
* Counting Problem
* <p>
* This problem can be solved by using DP.
* Please pay attention to the following cases:
* <p>
* 1. 0xxx is invalid.
* 2. * cannot match 0.
*/
public class _639 {
public int numDecodings(String s) {
if (s.startsWith("0")) return 0; // This is evil...
int mod = 1000000007;
int[] f = new int[s.length()];
for (int i = 0; i < s.length(); i++) {
for (int j = 0; j < 10; j++) {
// Note that * cannot match 0
if (s.charAt(i) - '0' == j || (j > 0 && s.charAt(i) == '*')) {
// Decode one digit
if (j > 0) {
f[i] = (f[i] + (i - 1 >= 0 ? f[i - 1] : 1)) % mod;
}
// Decode two digit
if (i > 0) {
for (int k = 1; k <= 2; k++) {
int val = k * 10 + j;
boolean valInRange = val >= 10 && val <= 26;
boolean kIsValid = s.charAt(i - 1) - '0' == k || s.charAt(i - 1) == '*';
if (valInRange && kIsValid) {
f[i] = (f[i] + (i - 2 >= 0 ? f[i - 2] : 1)) % mod;
}
}
}
}
}
}
return f[s.length() - 1];
}
}