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给定一个二叉树，返回该二叉树层序遍历的结果，（从左到右，一层一层地遍历）

AC的代码(Java版)：

import java.util.ArrayList;import java.util.Arrays;import java.util.LinkedList;class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val, TreeNode left, TreeNode right) {        this.val = val;        this.left = left;        this.right = right;    }}public class Solution {    public ArrayList
   
    
     > levelOrder(TreeNode root) {        ArrayList
     
      
       > answer = new ArrayList<>();        if (root == null)            return answer;        LinkedList
       
         treeNodes = new LinkedList<>();        treeNodes.add(root);        while (!treeNodes.isEmpty()) {            int levelSize = treeNodes.size();            ArrayList
        
          levelValues = new ArrayList<>(); for (int i = 0; i < levelSize; i++) { TreeNode currentNode = treeNodes.poll(); //get and remove the first one levelValues.add(currentNode.val); if (currentNode.left != null) treeNodes.add(currentNode.left); //add to the end if (currentNode.right != null) treeNodes.add(currentNode.right); } answer.add(levelValues); //遍历完一层 } return answer; } public static void printAnswer(ArrayList
         
          
           > answer) { int rowSum = answer.size(); for(int rowIndex = 0; rowIndex < rowSum; rowIndex++) { System.out.println(Arrays.toString(answer.get(rowIndex).toArray())); } } public static void main(String[] args) { TreeNode node5 = new TreeNode(5, null, null); TreeNode node4 = new TreeNode(4, null, null); TreeNode node3 = new TreeNode(3, node4, node5); TreeNode node2 = new TreeNode(2, null, null); TreeNode node1 = new TreeNode(1, node2, node3); printAnswer(new Solution().levelOrder(node1)); }}
          
         
        
       
      
     
    
   

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