diff --git a/src/main/java/com/fishercoder/solutions/_150.java b/src/main/java/com/fishercoder/solutions/_150.java
index c8e7abfbd9..c03cc3bd49 100644
--- a/src/main/java/com/fishercoder/solutions/_150.java
+++ b/src/main/java/com/fishercoder/solutions/_150.java
@@ -62,4 +62,30 @@ public int evalRPN(String[] tokens) {
         return Integer.parseInt(stack.pop());
     }
 
+    //using one stack only.
+    public int evalRPN1(String[] tokens) {
+        Stack<Integer> stack =  new Stack<Integer>();
+        for (int i = 0; i < tokens.length; i++) {
+            if (tokens[i].equals("+")) {
+                stack.push(stack.pop() + stack.pop());
+            }
+            else if (tokens[i].equals("-")) {
+                stack.push(-stack.pop() + stack.pop());
+            }
+            else if (tokens[i].equals("*") ) {
+                stack.push(stack.pop() * stack.pop());
+
+            }
+            else if (tokens[i].equals("/")) {
+                int a = stack.pop();
+                int b = stack.pop();
+                stack.push(b / a);
+
+            } else {
+                stack.push(Integer.parseInt (tokens[i]));
+            }
+        }
+        return stack.pop();
+    }
+
 }
diff --git a/src/main/java/com/fishercoder/solutions/_36.java b/src/main/java/com/fishercoder/solutions/_36.java
index e9eac998a8..a8d52374da 100644
--- a/src/main/java/com/fishercoder/solutions/_36.java
+++ b/src/main/java/com/fishercoder/solutions/_36.java
@@ -1,5 +1,5 @@
 package com.fishercoder.solutions;
-
+import java.util.*;
 /**Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.
 
  The Sudoku board could be partially filled, where empty cells are filled with the character '.'.
@@ -84,6 +84,21 @@ boolean isValidSquare(char[][] board, int row, int col) {
         return true;
     }
 
+    // the 3sets version
+    public boolean isValidSudoku1(char[][] board) {
+        for(int i = 0; i < 9; i++) {
+        Set<Character> rows = new HashSet<>();
+        Set<Character> cols = new HashSet<>();
+        Set<Character> cube = new HashSet<>();
+        for (int j = 0;j < 9; j++) {
+            if(board[i][j] != '.' && ! rows.add(board[i][j])) return false;
+            if(board[j][i] != '.' && ! cols.add(board[j][i])) return false;
+            if(board[3*(i/3) + j/3][3*(i%3) + j % 3] != '.' && !cube.add(board[3*(i/3) + j/3][3*(i%3) + j % 3] )) return false;
+        }
+    }
+        return true;
+}
+
     public static void main(String... strings) {
         _36 test = new _36();
         // char[][] board = new char[][]{
@@ -126,5 +141,6 @@ public static void main(String... strings) {
         // ["....5..1.",".4.3.....",".....3..1","8......2.","..2.7....",".15......",".....2...",".2.9.....","..4......"]
 
         System.out.println(test.isValidSudoku(board));
+        System.out.println(test.isValidSudoku1(board));
     }
 }
diff --git a/src/main/java/com/fishercoder/solutions/_659.java b/src/main/java/com/fishercoder/solutions/_659.java
new file mode 100644
index 0000000000..04eee49b1e
--- /dev/null
+++ b/src/main/java/com/fishercoder/solutions/_659.java
@@ -0,0 +1,4 @@
+package com.fishercoder.solutions;
+
+public class _659 {
+}