LeetCode-144二叉树的前序遍历(Binary Tree Preorder Traversal)

class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode(int x) { val = x; }
}
class Solution {
    List<Integer> preorder=new ArrayList<Integer>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if (root==null)
        	return preorder;
        preorder.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return preorder;
    }
}