102. Binary Tree Level Order Traversal

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 as:

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

confused what "{1,#,2,3}" means?

OJ’s Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where ‘#’ signifies a path terminator where no node exists below.

Here’s an example:

   1
  / \
 2   3
    /
   4
    \
     5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

// BFS 模板方法

public class Solution {
    public List<List> levelOrder(TreeNode root) {
        List<List> res = new ArrayList<>();
        Queue queue = new LinkedList();
        if(root == null){
            return res;
        }
        queue.offer(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            ArrayList tempRes = new ArrayList();
            for(int i = 0; i < size; i++){
                TreeNode curNode = queue.poll();
                if(curNode.left != null){
                    queue.offer(curNode.left);
                }
                if(curNode.right != null){
                    queue.offer(curNode.right);
                }
                tempRes.add(curNode.val);
            }
            res.add(new ArrayList(tempRes));
        }
        
        return res;
    }
}