0144 problem added

java/0144-binary-tree-preorder-traversal.java

@@ -0,0 +1,39 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+
+    static void preOrder(TreeNode root, List<Integer> res){
+
+
+        if(root == null){
+            return;
+        }
+        res.add(root.val);
+        preOrder(root.left,res);
+        preOrder(root.right,res);
+
+    }
+
+    public List<Integer> preorderTraversal(TreeNode root) {
+
+        List<Integer> res = new ArrayList<>();
+
+        preOrder(root,res);
+
+        return res;
+
+    }
+}