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BinaryTreeLevelOrderTraversal.java
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BinaryTreeLevelOrderTraversal.java
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package com.hncboy;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
* @author hncboy
* @date 2019/10/5 12:49
* @description 102.二叉树的层次遍历
*
* 给定一个二叉树,返回其按层次遍历的节点值。 (即逐层地,从左到右访问所有节点)。
*
* 例如:
* 给定二叉树: [3,9,20,null,null,15,7],
*
* 3
* / \
* 9 20
* / \
* 15 7
* 返回其层次遍历结果:
* [
* [3],
* [9,20],
* [15,7]
* ]
*/
public class BinaryTreeLevelOrderTraversal {
public static void main(String[] args) {
TreeNode node = new TreeNode(3);
node.left = new TreeNode(9);
node.right = new TreeNode(20);
node.right.left = new TreeNode(15);
node.right.right = new TreeNode(7);
System.out.println(new BinaryTreeLevelOrderTraversal().levelOrder(node));
}
private List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> ll = new ArrayList<>();
if (root == null) {
return ll;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
while (!queue.isEmpty()) {
// count 为该层节点的数量
int count = queue.size();
// 存放该层节点的值
List<Integer> list = new ArrayList<>();
// 遍历该层节点,将有左右节点的插入的队列中
while (count > 0) {
TreeNode node = queue.poll();
list.add(node.val);
if (node.left != null) {
queue.add(node.left);
}
if (node.right != null) {
queue.add(node.right);
}
count--;
}
ll.add(list);
}
return ll;
}
private static class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) {
val = x;
}
}
}