Solution.java

package ds.bfs.leetcode103;

import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/**
 * 二叉树的锯齿形层序遍历
 * LeetCode 103 https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/
 *
 * @author yangyi 2021年04月30日18:53:44
 */
public class Solution {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        if (root == null) {
            return new LinkedList<>();
        }
        List<List<Integer>> result = new LinkedList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int level = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            LinkedList<Integer> levelList = new LinkedList<>();
            for (int i = 0; i < size; i++) {
                TreeNode cur = queue.poll();
                if (cur != null) {
                    if (level % 2 == 0) {
                        levelList.addLast(cur.val);
                    } else {
                        levelList.addFirst(cur.val);
                    }
                }
                if (cur.left != null) {
                    queue.offer(cur.left);
                }
                if (cur.right != null) {
                    queue.offer(cur.right);
                }
            }
            result.add(levelList);
            level++;
        }
        return result;
    }

    public static void main(String[] args) {
        TreeNode node_3 = new TreeNode(3);
        TreeNode node_9 = new TreeNode(9);
        TreeNode node_20 = new TreeNode(20);
        TreeNode node_15 = new TreeNode(15);
        TreeNode node_7 = new TreeNode(7);

        node_3.left = node_9;
        node_3.right = node_20;
        node_20.left = node_15;
        node_20.right = node_7;

        Solution zigzagLevelOrder = new Solution();
        List<List<Integer>> result = zigzagLevelOrder.zigzagLevelOrder(node_3);
        for (List<Integer> integers : result) {
            for (Integer integer : integers) {
                System.out.print(integer + " ");
            }
            System.out.println();
        }
    }
}