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leetCode-102:Binary Tree Level Order Traversal

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问题描述

给定一个二叉树，要求按层级遍历输出结果。题目链接：**点我**

样例输入输出

输入：root = [3,9,20,null,null,15,7]

表示二叉树

3

/\

9 20

​ /\

15 7

输出：[[3],[9,20],[15,7]]

输入：root = [1]

输出：[[1]]

问题解法

用 BFS 遍历输出，代码如下

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/**
 * 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>> levelOrder(TreeNode root)
    {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        List<List<Integer>> result = new LinkedList<>();
        while (!queue.isEmpty())
        {
            if (root == null)
            {
                return new ArrayList<>();
            }
            
            List<Integer> temp = new LinkedList<>();
            for (int i = queue.size(); i > 0; i--)
            {
                TreeNode node = queue.poll();
                temp.add(node.val);
                
                if (node.left != null)
                {
                    queue.offer(node.left);
                }
                
                if (node.right != null)
                {
                    queue.offer(node.right);
                }
            }
            
            result.add(temp);
        }
        
        return result;
    }
}