103. Binary Tree Zigzag Level Order Traversal (M)

Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Constraints:

• The number of nodes in the tree is in the range [0, 2000].

• -100 <= Node.val <= 100

Solution:

Version 1:

这题和 102. 二叉树的层序遍历 几乎是一样的，只要用一个布尔变量 flag 控制遍历方向即可。

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        if (root == null) {
            return res;
        }

        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        // 为 true 时向右，false 时向左
        boolean flag = true;

        // while 循环控制从上向下一层层遍历
        while (!q.isEmpty()) {
            int sz = q.size();
            // 记录这一层的节点值
            LinkedList<Integer> level = new LinkedList<>();
            // for 循环控制每一层从左向右遍历
            for (int i = 0; i < sz; i++) {
                TreeNode cur = q.poll();
                // 实现 z 字形遍历
                if (flag) {
                    level.addLast(cur.val);
                } else {
                    level.addFirst(cur.val);
                }
                if (cur.left != null)
                    q.offer(cur.left);
                if (cur.right != null)
                    q.offer(cur.right);
            }
            // 切换方向
            flag = !flag;
            res.add(level);
        }
        return res;
    }
}

Version 2:

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result=new ArrayList<>();
        if(root==null){
            return result;
        }

        LinkedList<TreeNode> queue=new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> level=new ArrayList<Integer>();
            int size=queue.size();
            for(int i=0;i<size;i++){

                TreeNode current=queue.poll();
                level.add(current.val);
                if(current.left!=null){
                    queue.offer(current.left);
                }
                if(current.right!=null){
                    queue.offer(current.right);
                }
            }
            result.add(level);
        }
        
        for(int j = 0; j< result.size(); j++)
        {
            if(j%2 == 1)
            {
                Collections.reverse(result.get(j));
            }
        }
        return result;
    }
}