之字形打印二叉树

• 采用宽度遍历，利用队列的先进先出的特性，先让根结点入队，每次出队前计算当前队列的元素个数，然后将该元素个数的元素全部出队即为当前层的所有元素。设置标志位，每次遍历完取反即可让其以之字形方式存储。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> printFromTopToBottom(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        if (root == null)
            return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean zigzag = true;
        while (!queue.isEmpty()){
            int size = queue.size();
            List<Integer> level = new LinkedList<>();
            while (size > 0){
                TreeNode p = queue.poll();
                if (p.left != null)
                    queue.offer(p.left);
                if (p.right != null)
                    queue.offer(p.right);
                if (zigzag)
                    level.add(p.val);
                else
                    level.add(0,p.val);
                size--;
            }
            zigzag = !zigzag;
            res.add(level);
        }
        return res;
    }
}