二叉树Z字形遍历

/**
 * 二叉树Z型遍历
 */
public class ZTraverse {
    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

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

    public static List<List<Integer>> traverse(TreeNode root) {
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean leftToRight = false;
        List<List<Integer>> res = new ArrayList<>();
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> midList = new ArrayList<>();
            // 注意这里取top的时候是不一样的
            while (size-- > 0) {
                TreeNode top;
                //System.out.println(top.val);
                if (leftToRight) {
                    top = queue.poll();
                    if (top.left != null) queue.offer(top.left);
                    if (top.right != null) queue.offer(top.right);
                } else {
                    top = queue.pollLast();
                    if (top.right != null) queue.offer(top.right);
                    if (top.left != null) queue.offer(top.left);
                }
                midList.add(top.val);
            }
            res.add(midList);
            leftToRight = !leftToRight;
        }
        return res;
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.right.right = new TreeNode(5);
        List<List<Integer>> res = traverse(root);
        System.out.println(res);
    }
}