package io.github.dunwu.algorithm.tree.btree;
import io.github.dunwu.algorithm.tree.TreeUtils;
import org.junit.jupiter.api.Assertions;
import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
/**
* <code>二叉树的层次遍历 II</code> 算法实现
*
* <pre>
* 给定一个二叉树，返回其节点值自底向上的层次遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）
*
* 例如：
* 给定二叉树 [3,9,20,null,null,15,7],
*
* 3
* / \
* 9 20
* / \
* 15 7
* 返回其自底向上的层次遍历为：
*
* [
* [15,7],
* [9,20],
* [3]
* ]
* </pre>
*
* @see <a href="https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/">二叉树的层次遍历 II</a>
*/
public class 二叉树的层次遍历2 {
public static void main(String[] args) {
TreeNode tree = TreeUtils.buildTree(3, 9, 20, null, null, 15, 7);
List<List<Integer>> resultList = levelOrderBottom(tree);
List<List<Integer>> expectList = new LinkedList<>();
expectList.add(Arrays.asList(15, 7));
expectList.add(Arrays.asList(9, 20));
expectList.add(Arrays.asList(3));
Assertions.assertArrayEquals(expectList.toArray(), resultList.toArray());
}
public static List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> result = new LinkedList<>();
LinkedList<TreeNode> queue = new LinkedList<>();
if (root == null) return result;
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
List<Integer> list = new LinkedList<>();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
if (node != null) {
list.add(node.val);
if (node.left != null) queue.add(node.left);
if (node.right != null) queue.add(node.right);
}
}
result.add(list);
}
Collections.reverse(result);
return result;
}
}
