LeetCode

LeetCode - Easy - 107. Binary Tree Level Order Traversal II

Topic

• Tree
• Breadth-first Search

Description

https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example: Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

Analysis

方法一：BFS

方法二：DFS

Submission

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;

import com.lun.util.BinaryTree.TreeNode;

public class BinaryTreeLevelOrderTraversalII {
	
	//方法一：BFS
	public List<List<Integer>> levelOrderBottom1(TreeNode root) {
		List<List<Integer>> result = new LinkedList<>();
		if (root == null)
			return result;
		TreeNode p = root;
		LinkedList<TreeNode> queue = new LinkedList<>(Arrays.asList(p));

		while (!queue.isEmpty()) {
			int size = queue.size();
			List<Integer> list = new ArrayList<>();

			while (size-- > 0) {
				p = queue.poll();

				if (p.left != null) {
					queue.offer(p.left);
				}

				if (p.right != null) {
					queue.offer(p.right);
				}

				list.add(p.val);
			}
			result.add(0, list);
		}

		return result;
	}

	//方法二：DFS
	public List<List<Integer>> levelOrderBottom2(TreeNode root) {
		List<List<Integer>> wrapList = new LinkedList<List<Integer>>();
		levelOrderBottom2(wrapList, root, 0);
		return wrapList;
	}

	public void levelOrderBottom2(List<List<Integer>> list, TreeNode root, int level) {
		if (root == null)
			return;
		if (level >= list.size()) {
			list.add(0, new LinkedList<Integer>());
		}
		levelOrderBottom2(list, root.left, level + 1);
		levelOrderBottom2(list, root.right, level + 1);
		list.get(list.size() - level - 1).add(root.val);
	}
}

Test

import static org.junit.Assert.*;
import static org.hamcrest.CoreMatchers.*;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.junit.Test;

import com.lun.util.BinaryTree.TreeNode;

public class BinaryTreeLevelOrderTraversalIITest {

	@Test
	public void test() {
		BinaryTreeLevelOrderTraversalII obj = new BinaryTreeLevelOrderTraversalII();
		List<List<Integer>> expected = new ArrayList<>(
				Arrays.asList(Arrays.asList(15, 7), Arrays.asList(9, 20), Arrays.asList(3)));

		TreeNode root = new TreeNode(3);

		root.left = new TreeNode(9);
		root.right = new TreeNode(20);

		root.right.left = new TreeNode(15);
		root.right.right = new TreeNode(7);

		assertThat(obj.levelOrderBottom1(root), is(expected));
		assertThat(obj.levelOrderBottom2(root), is(expected));
	}
}

来源：oschina

链接：https://my.oschina.net/jallenkwong/blog/4807222