Leetcode-102 Binary Tree Level Order Traversal (layered Traversal of Binary Trees), leetcodetraversal

Leetcode-102 Binary Tree Level Order Traversal (layered Traversal of Binary Trees), leetcodetraversal
Binary Tree Level Order Traversal Total Accepted: 51429 Total Submissions: 174478
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
Return its level order traversal:
[
[3],
[9, 20],
[15, 7]
]
Confused what "{1, #, 2, 3}" means? > Read more on how binary tree is serialized on OJ.


Hide Tags: Tree Breadth-first Search (Breadth-first traversal)


Solution:
It traverses the breadth and records each layer. Breadth traversal is implemented by queue and record is implemented by list

Use a queue and a list. Queues are used to record each layer of nodes. The list is used to store nodes of each layer.

/** * Definition for a binary tree node. * public class TreeNode  * { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */

Public List <Integer> levelOrder (TreeNode root) {List <Integer> list = new partition List <Integer> (); // if the root node is empty, listif (root = null) {return list;} // create a Queue <TreeNode>, used to store all the node Queue <TreeNode> currentlevel = new Queue list <TreeNode> (); currentlevel. add (root); while (! Currentlevel. isEmpty () {// create a List <Integer> to record the Value List of all nodes in the current layer <Integer> currentList = new unknown List <Integer> (); // size indicates the number of nodes at the current layer. int size = currentlevel. size (); for (int I = 0; I <size; I ++) {// poll () Get and remove the header of this list (the first element) treeNode currentNode = currentlevel. poll (); // record the node values of each layer to currentList in List <Integer>. add (currentNode. val); if (currentNode. left! = Null) {currentlevel. add (currentNode. left);} if (currentNode. right! = Null) {currentlevel. add (currentNode. right) ;}}// Add the current List <Integer> to List <Integer>. add (currentList);} return list ;}