LeetCode 103. 二叉树的锯齿形层序遍历

给定一个二叉树，返回其节点值的锯齿形层序遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

BFS+双端队列

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> ans = new ArrayList<>();
        if(root == null){
            return ans;   
        }
        queue.offer(root);
        boolean isLeftToRight = true;
        while(!queue.isEmpty()){
            Deque<Integer> deque = new LinkedList<>();
            int size = queue.size();
            for(int i = 0; i < size; i++){
                TreeNode cur = queue.poll();
                if(isLeftToRight){
                    deque.offerLast(cur.val);
                }else{
                    deque.offerFirst(cur.val);
                }
                if(cur.left != null){
                    queue.offer(cur.left);
                }
                if(cur.right != null){
                    queue.offer(cur.right);
                }
            }
            isLeftToRight = !isLeftToRight;
            ans.add(new ArrayList(deque));
        }
        return ans;
        
    }
}