Add leetcode 639.

1 files changed, 56 insertions, 0 deletions

diff --git a/leetcode/cpp/639.cpp b/leetcode/cpp/639.cpp
new file mode 100644
index 0000000..ead252e
--- /dev/null
+++ b/leetcode/cpp/639.cpp
@@ -0,0 +1,56 @@
+#include <string>
+
+using std::string;
+
+const long long M = 1e9 + 7;
+
+class Solution {
+public:
+  long long f[100010]{0};
+
+  int numDecodings(string s) {
+    f[0] = 1;
+    if (s.front() == '*') {
+      f[1] = 9;
+    } else if (s.front() == '0') {
+      f[1] = 0;
+    } else {
+      f[1] = 1;
+    }
+
+    for (int i = 2; i <= s.size(); i++) {
+      char c = s[i - 1], l = s[i - 2];
+      if (c == '*') {
+        f[i] += f[i - 1] * 9;
+        if (l == '*') {
+          f[i] += f[i - 2] * 15;
+        } else if (l == '1') {
+          f[i] += f[i - 2] * 9;
+        } else if (l == '2') {
+          f[i] += f[i - 2] * 6;
+        }
+        f[i] %= M;
+      } else if (c == '0') {
+        if (l == '1' || l == '2')
+          f[i] += f[i - 2];
+        else if (l == '*')
+          f[i] += f[i - 2] * 2;
+        f[i] %= M;
+      } else {
+        f[i] += f[i - 1];
+        if (l == '*') {
+          if (c <= '6')
+            f[i] += f[i - 2] * 2;
+          else
+            f[i] += f[i - 2];
+        } else if (l == '1')
+          f[i] += f[i - 2];
+        else if (l == '2' && c <= '6')
+          f[i] += f[i - 2];
+        f[i] %= M;
+      }
+    }
+
+    return f[s.size()] % M;
+  }
+};