二叉树之字形层序遍历

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();

        if(root == null) return res;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);

        boolean level = false;

        while(!queue.isEmpty()){
            ArrayList<Integer> list = new ArrayList<>();
            int size = queue.size();
            for(int i = 0; i < size; ++i){
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left != null){queue.add(node.left);}
                if(node.right != null){queue.add(node.right);}
            }
            if(level){
                Collections.reverse(list);
            }
            level = !level;
            res.add(list);
        }
        return res;
    }
}