二叉树遍历

class TreeNode{
    public String val;
    public TreeNode left;
    public TreeNode right;

    public TreeNode(String val) {
        this.val = val;
    }

    public TreeNode(String val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}