add numDecodings.cpp

Author: kamyu104

C++/numDecodings.cpp

@@ -0,0 +1,25 @@
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+class Solution {
+    public:
+        int numDecodings(string s) {
+            if(s.empty()) return 0;
+
+            int prev = 0; // f[n - 2]
+            int cur = 1;  // f[n - 1]
+
+            for(int i = 1; i <= s.length(); ++i) {
+                if(s[i - 1] == '0')
+                    cur = 0; // f[n - 1] = 0
+                if(i < 2 || !(s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6')))
+                    prev = 0; // f[n - 2] = 0;
+
+                int tmp = cur;
+                cur += prev; // f[n] = f[n - 1] + f[n - 2]
+                prev = tmp;
+            }
+
+            return cur;
+        }
+};