diff --git a/Easy/Pascal_Triangle.java b/Easy/Pascal_Triangle.java
new file mode 100644
index 00000000..2503b32e
--- /dev/null
+++ b/Easy/Pascal_Triangle.java
@@ -0,0 +1,55 @@
+import java.util.*;
+/**
+ * Pascal's Triangle
+ * Given numRows, generate the first numRows of Pascal's triangle.
+ * For example, given numRows = 5,
+ * Return
+ * [
+ * [1],
+ * [1,1],
+ * [1,2,1],
+ * [1,3,3,1],
+ * [1,4,6,4,1]
+ * ]
+ * Tags: Array
+ * Similar Problems: Pascal's Triangle II
+ *
+ * Analysis : using dp
+ * @author chenshuna
+ */
+
+public class Pascal_Triangle {
+    public static List<List<Integer>> generate(int numRows) {
+        List<List<Integer>> res = new ArrayList<List<Integer>>();
+        ArrayList<Integer> member = new ArrayList<Integer>();
+        if(numRows <= 0){
+            return res;
+        }
+        member.add(1);
+        res.add( member);
+        return generateArray(res, numRows-1);
+    }
+    
+    public static List<List<Integer>> generateArray(List<List<Integer>> res, int n){
+        if(n == 0){
+            return res;
+        }
+        else{
+            List<Integer> resLast= new ArrayList<Integer>(); 
+            ArrayList<Integer> newMember = new ArrayList<Integer>(); 
+            resLast = res.get(res.size()-1);
+            newMember.add(1);
+            for(int i = 0; i < resLast.size()-1; i++){
+               int temp = resLast.get(i) + resLast.get(i+1);
+               newMember.add(temp);
+            }
+            newMember.add(1);
+            res.add(newMember);
+            return generateArray(res, n-1);
+        }
+    }
+
+    public static void main(String[] args) {
+        System.out.print(generate(5));
+    }
+}
diff --git a/Medium/BinaryTreePreorderTraversal.java b/Medium/BinaryTreePreorderTraversal.java
index b80aaf85..614ed953 100644
--- a/Medium/BinaryTreePreorderTraversal.java
+++ b/Medium/BinaryTreePreorderTraversal.java
@@ -1,25 +1,25 @@
-package gitLeetCode;
-
 import java.util.*;
 
 /**
  * Given a binary tree, return the preorder traversal of its nodes' values.
-
-   For example:
-   Given binary tree {1,#,2,3},
-   1
-    \
-     2
-    /
-   3
-   return [1,2,3].
- * @author chenshuna
+ * For example:
+ *  Given binary tree {1,#,2,3},
+ * 1
+ *  \
+ *   2
+ *  /
+ *  3
+ * return [1,2,3].
+ * Tags:  Tree, Stack
+ * Similar Problems: Binary Tree Inorder Traversal, Verify Preorder Sequence in Binary Search Tree
  *
  * Note: Recursive solution
+ * @author chenshuna
  */
 
 public class BinaryTreePreorderTraversal {
-	/**
+
+    /**
     * Recursive solution
     * @param args
     */
@@ -31,6 +31,7 @@ public static List<Integer> preorderTraversal(TreeNode root) {
         preorderTraversalTree(root, res);
         return res;
     }
+
     public static void preorderTraversalTree(TreeNode root, List<Integer> res){
         if(root != null){
             res.add(root.val);
@@ -66,16 +67,14 @@ public static List<Integer> preorderTraversalIterative(TreeNode root) {
     }
     
     public static void main(String[] args) {
-		// TODO Auto-generated method stub
-		TreeNode res = new TreeNode(0);
-		res.left = new TreeNode(2);
-		res.right = new TreeNode(1);
-		res.left.left = new TreeNode(4);
-		res.left.right = new TreeNode(5);
-		res.left.left.left = new TreeNode(9);
-		
-		System.out.print(preorderTraversal(res));
-		System.out.print(preorderTraversalIterative(res));
-	}
+        TreeNode res = new TreeNode(0);
+        res.left = new TreeNode(2);
+        res.right = new TreeNode(1);
+        res.left.left = new TreeNode(4);
+        res.left.right = new TreeNode(5);
+        res.left.left.left = new TreeNode(9); 
+        System.out.print(preorderTraversal(res));
+        System.out.print(preorderTraversalIterative(res));
+    }
 
 }
diff --git a/Medium/RemoveDuplicatesfromSortedArray2.java b/Medium/RemoveDuplicatesfromSortedArray2.java
new file mode 100644
index 00000000..d90e2a92
--- /dev/null
+++ b/Medium/RemoveDuplicatesfromSortedArray2.java
@@ -0,0 +1,44 @@
+/**
+ * Remove Duplicates from Sorted Array II
+ * Follow up for "Remove Duplicates":
+ * What if duplicates are allowed at most twice?
+ * For example,
+ * Given sorted array nums = [1,1,1,2,2,3],
+ * Your function should return length = 5, with the first five elements of nums being 1, 1, 2, 2 and 3. It doesn't matter what you leave beyond the new length.
+ * Tag: Linkedlist
+ *
+ * Note: create a countEach variable to count duplicated times for each integer in array.
+ * Time complexity = O(n) 
+ * @author chenshuna
+ */
+
+public class RemoveDuplicatesfromSortedArray2 {
+    public static int removeDuplicates(int[] nums) {
+        int res = 1;
+        if(nums.length <= 1){
+            return nums.length;
+        }
+        int countEach = 1;
+        for(int i = 1; i<nums.length; i++){
+            if(nums[i] == nums[i-1] && countEach < 2){
+                nums[res] = nums[i];
+                countEach++;
+                res++;
+            }
+            else if(nums[i] != nums[i-1]){
+                nums[res] = nums[i];
+                res++;
+                countEach = 1;
+            }
+            else{
+                countEach++;
+            }
+        }
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] res = {1,1,1,2,3}; 
+        System.out.print(removeDuplicates(res));
+    }
+}