Create Binary Tree Preorder Traversal

Author: HarryWangATX

Binary Tree Preorder Traversal

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+/**
+ * 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 {
+    List<Integer> pre = new ArrayList<>();
+    public List<Integer> preorderTraversal(TreeNode root) {
+        preO(root);
+        return pre;
+    }
+    public void preO(TreeNode root){
+        if(root == null){
+            return ;
+        }
+        pre.add(root.val);
+        preO(root.left);
+        preO(root.right);
+    }
+}