added solution for leetcode/dynamic programming/91.cpp

Author: ganm0

leetcode/cpp/dynamic programming/91.cpp

@@ -0,0 +1,38 @@
+//Decode ways
+
+class Solution {
+public:
+    bool check(string s, int i){
+        int num = (s[i-1] - '0')*10 + (s[i] - '0');
+        if(num>=10 && num<=26)return true;
+        return false; 
+    }
+    
+    int numDecodings(string s) {
+        if(s.length() == 0)return 0;
+        vector<int> dp(s.length()+1, 0);
+        dp[0] = 1;
+        if(s[0]>'0'){
+            dp[1] = 1;
+        }
+        else {
+            return 0;
+        }
+        if(s.length() == 1)return s.length();
+        for(int i=1;i<s.length();i++){
+            if(s[i] == '0' && s[i-1]>'0' && s[i-1]<'3'){
+                dp[i+1] = dp[i-1];
+            }
+            else if(s[i] == '0'){
+                return 0;
+            }
+            else if(check(s, i)){
+                dp[i+1] = dp[i-1] + dp[i];
+            }
+            else{
+                dp[i+1] = dp[i];
+            }
+        }
+        return dp[s.length()];
+    }
+};