剑指offer-27.从上往下打印二叉树（层次遍历）(171)

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27.从上往下打印二叉树（层次遍历）(171)

• 题目描述：从上往下打印出二叉树的每个节点，同层节点从左至右打印。

• 代码：

  package _28.从上往下打印二叉树;
  
  import java.util.ArrayList;
  import java.util.LinkedList;
  import java.util.Queue;
  /**
   * 题目描述：从上往下打印出二叉树的每个节点，同层节点从左至右打印。
   * @author Administrator
   *
   */
  public class PrintFromTopToBottom {
      public ArrayList<Integer> printFromTopToBottom(TreeNode root) {
          ArrayList<Integer> list = new ArrayList<>();
          if(root == null) 
          	return list;
          
          //利用队列来保存根元素，以便按顺序访问其孩子节点
          Queue<TreeNode> queue = new LinkedList<>();
          TreeNode node = root;
          queue.add(node);
          while(!queue.isEmpty()){
          	node = queue.remove();
          	list.add(node.val);
          	if(node.left != null){
          		queue.add(node.left);
          	}
          	if(node.right != null){
          		queue.add(node.right);
          	}
          }
          return list;
      }
      
      public static void main(String[] args) {
      	TreeNode root = new TreeNode(0);
  		TreeNode node1 = new TreeNode(1);
  		TreeNode node2 = new TreeNode(2);
  		TreeNode node3 = new TreeNode(3);
  		TreeNode node4 = new TreeNode(4);
  		root.left = node1;
  		root.right = node2;
  		node1.left = node3;
  		node1.right = node4;
  		
  		PrintFromTopToBottom print = new PrintFromTopToBottom();
  		ArrayList<Integer> list = print.printFromTopToBottom(root);
  		System.out.println(list);
  	}
  }
  class TreeNode {
      int val = 0;
      TreeNode left = null;
      TreeNode right = null;
  
      public TreeNode(int val) {
          this.val = val;
  
      }
  }