leetcode103二叉树的锯齿形层序遍历

Posted 2021-07-05 lisin-lee-cooper

一.问题描述
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）。
例如：
给定二叉树 [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
返回锯齿形层序遍历如下：
[
[3],
[20,9],
[15,7]
]

二.示例代码

public class ZLevelBinarySearchTree103 {

    public static void main(String[] args) {
        TreeNode root = new TreeNode(5);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(9);
        TreeNode node3 = new TreeNode(7);
        TreeNode node4 = new TreeNode(10);
        root.left = node1;
        root.right = node2;
        node2.left = node3;
        node2.right = node4;
        List<List<Integer>> result = zLevelOrder(root);
        System.out.println(result);
    }

    private static List<List<Integer>> zLevelOrder(TreeNode root) {
        List<List<Integer>> ans = new LinkedList<>();
        if (root == null) {
            return ans;
        }

        Queue<TreeNode> nodeQueue = new LinkedList<>();
        nodeQueue.offer(root);
        boolean isOrderLeft = true;

        while (!nodeQueue.isEmpty()) {
            Deque<Integer> levelList = new LinkedList<>();
            int size = nodeQueue.size();
            for (int i = 0; i < size; ++i) {
                TreeNode curNode = nodeQueue.poll();
                if (isOrderLeft) {
                    levelList.offerLast(curNode.val);
                } else {
                    levelList.offerFirst(curNode.val);
                }
                if (curNode.left != null) {
                    nodeQueue.offer(curNode.left);
                }
                if (curNode.right != null) {
                    nodeQueue.offer(curNode.right);
                }
            }
            ans.add(new LinkedList<>(levelList));
            isOrderLeft = !isOrderLeft;
        }
        return ans;
    }

}