add prob #331(2nd); O(N) in time and O(1) in space;

Author: randomwalk10

331_Verify_Preorder_Serialization_of_a_Binary_Tree_2nd.cpp

@@ -0,0 +1,31 @@
+/* O(N) in time and O(1) in space */
+#include <string>
+using namespace std;
+
+class Solution {
+public:
+    bool isValidSerialization(string preorder) {
+        if(preorder.empty()) return false;
+        int res = 1;
+
+        int i=0;
+        while(i<(int)preorder.size()){
+            while( (i<(int)preorder.size())&&(preorder[i]!=',') ){
+                ++i;
+            }
+            if(preorder[i-1]=='#'){
+                --res;
+            }
+            else{
+                ++res;
+            }
+
+            if( (i<(int)preorder.size())&&(res==0) ){
+                return false;
+            }
+            ++i;
+        }
+
+        return res==0;
+    }
+};