decode ways solution

Author: dylan-jung

decode-ways/dylan-jung.cpp

@@ -0,0 +1,38 @@
+class Solution {
+public:
+    int mem[100];
+    string str;
+
+    bool isValid(string s) {
+        if(s.size() == 1) {
+            return '1' <= s[0] && s[0] <= '9';
+        }
+        else if(s.size() == 2) {
+            if(s[0] == '1') return '0' <= s[1] && s[1] <= '9';
+            else if(s[0] == '2') return '0' <= s[1] && s[1] <= '6';
+        }
+        return false;
+    }
+
+    int dfs(int idx) {
+        if(idx >= str.size()) return 1;
+
+        int& ret = mem[idx];
+        if(ret != -1) return ret;
+        ret = 0;
+
+        if (isValid(str.substr(idx, 1))){
+            ret += dfs(idx+1);
+        }
+        if (idx < str.size() - 1 && isValid(str.substr(idx, 2))){
+            ret += dfs(idx+2);
+        }
+        return ret;
+    }
+
+    int numDecodings(string s) {
+        str = s;
+        fill(mem, mem+100, -1);
+        return dfs(0);
+    }
+};