Add trees pre in post traversal java (#178)

* add pre, in and post traversal trees in java

* Update README.md

* update line 49 to pass lint test

change "System.out.print("\n\nin order: "); 
to 
System.out.println("\n");
System.out.print("in order: ");

trees/README.md

@@ -5,3 +5,7 @@
 1. [Pre, In & Post Order Traversals](c-or-cpp/pre-in-post-traversal.cpp)
 2. [Level Order Traversal](c-or-cpp/level-order-traversal.cpp)
 3. [Build Binary Tree using Pre,In & Post Order](c-or-cpp/build-binary-tree.cpp)
+
+### Java
+
+1. [Pre, In & Post Order Traversals](java/pre_in_post_traversal.java)

trees/java/pre_in_post_traversal.java

@@ -0,0 +1,76 @@
+class Node{  
+    int value;  
+    Node left;  
+    Node right;  
+
+    public Node(int value) {  
+        this.value = value;  
+    }  
+}
+
+public class pre_in_post_traversal{
+
+    public static void preOrder(Node node){
+        if (node == null) return;
+
+        System.out.print(node.value + ", ");
+        preOrder(node.left);
+        preOrder(node.right);
+    }
+
+    public static void inOrder(Node node){
+        if(node == null) return;
+
+        inOrder(node.left);
+        System.out.print(node.value + ", ");
+        inOrder(node.right);
+    }
+
+    public static void postOrder(Node node){
+        if(node == null) return;
+        postOrder(node.left);
+        postOrder(node.right);
+        System.out.print(node.value + ", ");
+    }
+
+
+    public static void main(String [] args){
+        Node root = new Node(4);
+        root.left = new Node(2);
+        root.right = new Node(7);
+        root.left.right = new Node(6);
+        root.left.left = new Node(1);
+        root.right.right = new Node(9);
+
+        System.out.print("pre order: ");
+        preOrder(root);
+        
+        System.out.println("\n");
+        System.out.print("in order: ");
+        inOrder(root);
+
+        System.out.print("\n\npost order: ");
+        postOrder(root);
+    }
+}
+
+/*
+tree structure:
+        4
+      /   \
+    2       7
+  /   \       \
+1      6       9
+
+to run the file:
+    javac .\pre_in_post_traversal.java
+    java pre_in_post_traversal
+
+results:
+    pre order: 4, 2, 1, 6, 7, 9, 
+
+    in order: 1, 2, 6, 4, 7, 9,
+
+    post order: 1, 6, 2, 9, 7, 4,
+
+*/