给你二叉树的根节点 root ，返回它节点值的 前序 遍历。
//leetcode submit region begin(Prohibit modification and deletion)
/**
* 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> results = new ArrayList<Integer>();
public List<Integer> preorderTraversal(TreeNode root) {
if(root == null){
return results;
}
results.add(root.val);
preorderTraversal(root.left);
preorderTraversal(root.right);
return results;
}
}
//leetcode submit region end(Prohibit modification and deletion)