Leetcode038--二叉树的层序遍历

一、原题

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level). 

For example: 

Given binary tree {3,9,20,#,#,15,7},

3
/ \
9  20
/  \
15   7

1
2
3
4
5
1
2
3
4
5

return its level order traversal as:

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

一、中文

给定一个二叉树，输出它的每一层的结点。

三、举例

上面的题目中给的就有例子，还是比较好理解的

四、思路

 

使用两个队列的方式，队列是先进先出的，所以我们这里使用两个队列，一个队列存储当前层的二叉树结点，另外一个队列来存储下一层的结点，然后将当前层的左右子结点按照顺序放入下一层中，处理完下一层之后将本层和下一层进行一个互换的操作。

五、程序

package code;

import java.util.Deque;
import java.util.Iterator;
import java.util.LinkedList;

//class TreeNode{
//	int val;
//	TreeNode left;
//	TreeNode right;
//	TreeNode(int x){
//		val = x;
//	}
//}

public class LeetCode51{
public static void main(String args[]){
TreeNode n1 = new TreeNode(1);
TreeNode n2 = new TreeNode(2);
TreeNode n3 = new TreeNode(2);
TreeNode n4 = new TreeNode(3);
TreeNode n5 = new TreeNode(4);
TreeNode n6 = new TreeNode(3);
TreeNode n7 = new TreeNode(4);

n1.left = n2;
n1.right = n3;

n2.left = n4;
n2.right = n5;

n3.left = n7;
n3.right = n6;

LinkedList<LinkedList<Integer>> res = levelOrder(n1);

Iterator<LinkedList<Integer>> it1 = res.iterator();
while(it1.hasNext()){
Iterator<Integer> it2 = it1.next().iterator();
while(it2.hasNext()){
System.out.print(it2.next()+" ");
}
System.out.println();
}
}

//第一种使用递归的方式
public static LinkedList<LinkedList<Integer>> levelOrder(TreeNode root) {
LinkedList<LinkedList<Integer>> res = new LinkedList<>();

if(root == null){
return res;
}

Deque<TreeNode> cur = new LinkedList<>();
Deque<TreeNode> sub = new LinkedList<>();
Deque<TreeNode> exc;

cur.addLast(root);
TreeNode node;

while(!cur.isEmpty()){
LinkedList<Integer> temp = new LinkedList<>();

while(!cur.isEmpty()){
node = cur.removeFirst();
temp.add(node.val);

if(node.left != null){
sub.addLast(node.left);
}

if(node.right != null){
sub.addLast(node.right);
}

}

exc = cur;
cur = sub;
sub = exc;

res.add(temp);

}
return res;
}

}

----------------------------------------output-------------------------------------------

1
2 2
3 4 4 3