103.二叉树的锯齿形层序遍历

方法：广度优先搜索

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        //利用广度优先搜索
        List<List<Integer>> ans = new LinkedList<List<Integer>>();
        if(root == null){
            return ans;
        }

        Queue<TreeNode> queue = new LinkedList<>();  //定义一个队列
        queue.offer(root);
        boolean isLeft = true;
        while(!queue.isEmpty()){
            Deque<Integer> levelList = new LinkedList<>();
            int size = queue.size();
            for(int i = 0 ;i < size;i++){
                TreeNode curNode = queue.poll();
                if(isLeft){
                    levelList.offerLast(curNode.val);
                }else{
                    levelList.offerFirst(curNode.val);
                }
                if(curNode.left!=null){
                    queue.offer(curNode.left);
                }
                if(curNode.right!=null){
                    queue.offer(curNode.right);
                }
            }
            ans.add(new LinkedList<Integer>(levelList));
            isLeft = !isLeft;
        }
        return ans;

    }
    
}