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
deleted file mode 100644
index 4f8d442b02..0000000000
--- a/src/main/java/com/fishercoder/solutions/_659.java
+++ /dev/null
@@ -1,61 +0,0 @@
-package com.fishercoder.solutions;
-
-import java.util.*;
-
-/**
- * 659. Split Array into Consecutive Subsequences
- *
- * You are given an integer array sorted in ascending order (may contain duplicates),
- * you need to split them into several subsequences,
- * where each subsequences consist of at least 3 consecutive integers. Return whether you can make such a split.
-
- Example 1:
- Input: [1,2,3,3,4,5]
- Output: True
- Explanation:
- You can split them into two consecutive subsequences :
- 1, 2, 3
- 3, 4, 5
-
- Example 2:
- Input: [1,2,3,3,4,4,5,5]
- Output: True
- Explanation:
- You can split them into two consecutive subsequences :
- 1, 2, 3, 4, 5
- 3, 4, 5
-
- Example 3:
- Input: [1,2,3,4,4,5]
- Output: False
-
- Note:
- The length of the input is in range of [1, 10000]
- */
-
-public class _659 {
-
-    /**reference: https://discuss.leetcode.com/topic/99187/java-o-n-time-o-n-space*/
-    public boolean isPossible(int[] nums) {
-        Map<Integer, Integer> freqMap = new HashMap<>();
-        for (int i : nums) {
-            freqMap.put(i, freqMap.getOrDefault(i, 0) + 1);
-        }
-        Map<Integer, Integer> appendFreqMap = new HashMap<>();
-        for (int i : nums) {
-            if (freqMap.get(i) == 0) {
-                continue;
-            } else if (appendFreqMap.getOrDefault(i, 0) > 0) {
-                appendFreqMap.put(i, appendFreqMap.get(i) - 1);
-                appendFreqMap.put(i+1, appendFreqMap.getOrDefault(i+1, 0) + 1);
-            } else if (freqMap.getOrDefault(i+1, 0) > 0 && freqMap.getOrDefault(i+2, 0) > 0) {
-                freqMap.put(i+1, freqMap.get(i+1) - 1);
-                freqMap.put(i+2, freqMap.get(i+2) - 1);
-                appendFreqMap.put(i+3, appendFreqMap.getOrDefault(i+3, 0) + 1);
-            } else return false;
-            freqMap.put(i, freqMap.get(i) - 1);
-        }
-        return true;
-    }
-
-}