二叉树 Z 字型遍历

考察点：遍历

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {        List<List<Integer>> res =new ArrayList<List<Integer>>();        linkedList<TreeNode> l = new linkedList<TreeNode>();        boolean flag = true;        if (root == null) { return res;        }        l.add(root);        while (l.size() != 0) { flag = !flag; int size = l.size(); List<Integer> list = new ArrayList<Integer>(); for (int i = 0; i < size; i++) {     TreeNode temp = l.remove();     list.add(temp.val);     if (temp.left != null)l.add(temp.left);     if (temp.right != null)l.add(temp.right); } if (flag) {     Collections.reverse(list); } res.add(list);        }        return res;    }