LeetCode 103. 二叉树的锯齿形层次遍历(Binary Tree Zigzag Level Order Traversal)

103. 二叉树的锯齿形层次遍历
103. Binary Tree Zigzag Level Order Traversal

题目描述
给定一个二叉树，返回其节点值的锯齿形层次遍历。（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）。

LeetCode103. Binary Tree Zigzag Level Order Traversal中等

例如:
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回锯齿形层次遍历如下:

[
  [3],
  [20,9],
  [15,7]
]

Java 实现

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

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

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

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        if (root == null) {
            return new LinkedList<>();
        }
        List<List<Integer>> res = new LinkedList<>();
        int count = 1;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new LinkedList<>();
            for (int i = 0; i < size; i++) {
                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 (count % 2 == 0) {
                Collections.reverse(list);
            }
            count++;
            res.add(list);
        }
        return res;
    }
}

相似题目

• 102. 二叉树的层次遍历 Binary Tree Level Order Traversal

参考资料

• https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/
• https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/