中序走訪結果: 4 2 5 1 3
TreeNode Object ( [val] => 1 [left] => TreeNode Object ( [val] => 2 [left] => TreeNode Object ( [val] => 4 [left] => [right] => ) [right] => TreeNode Object ( [val] => 5 [left] => [right] => ) ) [right] => TreeNode Object ( [val] => 3 [left] => [right] => ) )