# 103. Binary Tree Zigzag Level Order Traversal



## reverse list

T: O(n\*m), m is the number of reverse operations

S: O(n)

```java
/**
 * 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) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        int level = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            
            List<Integer> list = new ArrayList<>();
            for (int q = 0; q < size; q++) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            if (level % 2 != 0) { // odd level need reverse
                Collections.reverse(list);
            }
            res.add(list);
            level++;
        }
        return res;
    }
}

/*
BFS


*/
```



## reverse with addFirst()

use linkedlist as data store, so we can use addFirst()

T: O(n)

S: O(n)

```java
/**
 * 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) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        int level = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            
            LinkedList<Integer> list = new LinkedList<>();
            for (int q = 0; q < size; q++) {
                TreeNode node = queue.poll();
                
                if (level % 2 == 0) { 
                    list.add(node.val);
                } else { // odd level need reverse
                    list.addFirst(node.val);
                }
                
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            res.add(list);
            level++;
        }
        return res;
    }
}

/*
BFS


*/
```
