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test09.java
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package byteDance;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
* Created by lizeyang on 2020/5/12.
* 从上往下打印二叉树
*/
class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int val) {
this.val = val;
}
}
public class test09 {
//利用队列实现层次遍历，时间复杂度O(n)，空间复杂度O(n)
public static List<List<Integer>> test(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
int level = 0;
while (!queue.isEmpty()) {
int size = queue.size();
res.add(new ArrayList<>());
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
res.get(level).add(node.val);
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
level++;
}
return res;
}
public static void main(String[] args) {
TreeNode root = new TreeNode(5);
root.left = new TreeNode(4);
root.right = new TreeNode(8);
root.left.left = new TreeNode(3);
root.right.left = new TreeNode(7);
root.right.left.left = new TreeNode(6);
root.right.right = new TreeNode(9);
List<List<Integer>> res = test(root);
for (int i = 0; i < res.size(); i++) {
for (int j = 0; j < res.get(i).size(); j++) {
System.out.print(res.get(i).get(j) + " ");
}
System.out.println();
}
}
}