題目連結：103.二叉樹的鋸齒形層序遍歷

思路：雙棧。

程式碼：

/**
 * 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>> zigzagLevelOrder(TreeNode root) {
        
 
if(root == null) return new ArrayList<>();
        Deque<TreeNode> cur = new ArrayDeque<>();
        Deque<TreeNode> next = new ArrayDeque<>();
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        cur.push(root);
        
 
boolean reverse = false;
        while(!cur.isEmpty()){
            TreeNode node = cur.pop();
            list.add(node.val);
            if(reverse){
                if(node.right != null) next.push(node.right);
                if(node.left != null) next.push(node.left);
            }else{
                
 
if(node.left != null) next.push(node.left);
                if(node.right != null) next.push(node.right);
            }
            if(cur.isEmpty()){
                Deque<TreeNode> t = cur;
                cur = next;
                next = t;
                reverse = !reverse;
                res.add(list);
                list = new ArrayList<>();
            }
        }
        return res;
    }
}