题解 | #从上往下打印二叉树#
从上往下打印二叉树
http://www.nowcoder.com/practice/7fe2212963db4790b57431d9ed259701
import java.util.ArrayList;
import java.util.*;
/**
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
// 注意:队列适用于广度优先遍历
// 注意:不必出一进一,此例中就存在出一进二、出一进一、出一进零
// 时空复杂度O(n)
public class Solution {
public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
ArrayList<Integer> res = new ArrayList<>();
Queue<TreeNode> q = new ArrayDeque<>();
if (root==null)
return res;
q.add(root);
while (!q.isEmpty()){
TreeNode node = q.poll();
res.add(node.val);
if (node.left != null){
q.add(node.left);
}
if (node.right != null){
q.add(node.right);
}
}
return res;
}
}