第 32-1 题：不分行从上往下打印二叉树
传送门：不分行从上往下打印二叉树，牛客网 online judge 地址。
从上往下打印出二叉树的每个结点，同一层的结点按照从左到右的顺序打印。
样例：
输入如下图所示二叉树
[8, 12, 2, null, null, 6, null, 4, null, null, null]8 / \ 12 2 / 6 / 4输出：
[8, 12, 2, 6, 4]
思路：层序遍历，借助利用队列实现。其实就是层序遍历，使用的辅助的数据结构是“队列”，在遍历的过程中，注意到一些细节就可以了。在实现的过程中，要防止：
1、将空元素放入队列；
2、队列元素出队的时候，先判断队列是否为空，非空才可以出队。
Python 代码：
# Definition for a binary tree node.
class TreeNode(object):
def __init__(self, x):
self.val = x
self.left = None
self.right = None
class Solution:
def printFromTopToBottom(self, root):
"""
:type root: TreeNode
:rtype: List[int]
"""
if root is None:
return []
queue = [root]
res = []
while queue:
# 弹出队首元素，索引编号 0 不要忘记写了
top = queue.pop(0)
res.append(top.val)
if top.left:
queue.append(top.left)
if top.right:
queue.append(top.right)
return res
Java 代码：
import java.util.ArrayList;
import java.util.LinkedList;
class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
public class Solution {
public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
ArrayList<Integer> res = new ArrayList<>();
if (root == null) {
return res;
}
LinkedList<TreeNode> queue = new LinkedList<>();
queue.addLast(root);
while (!queue.isEmpty()) {
int curSize = queue.size();
for (int i = 0; i < curSize; i++) {
TreeNode curNode = queue.removeFirst();
res.add(curNode.val);
if (curNode.left != null) {
queue.addLast(curNode.left);
}
if (curNode.right != null) {
queue.addLast(curNode.right);
}
}
}
return res;
}
public static void main(String[] args) {
TreeNode node8 = new TreeNode(8);
TreeNode node6 = new TreeNode(6);
TreeNode node10 = new TreeNode(10);
TreeNode node5 = new TreeNode(5);
TreeNode node7 = new TreeNode(7);
TreeNode node9 = new TreeNode(9);
TreeNode node11 = new TreeNode(11);
node8.left = node6;
node8.right = node10;
node6.left = node5;
node6.right = node7;
node10.left = node9;
node10.right = node11;
Solution solution = new Solution();
ArrayList<Integer> printFromTopToBottom = solution.PrintFromTopToBottom(node8);
System.out.println(printFromTopToBottom);
}
}
