week 3 102. Binary Tree Level Order Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>(); 
        List<List<Integer>> wrapList = new LinkedList<List<Integer>>();
        
        if(root == null) 
        return wrapList;
        
        queue.offer(root);
       
        
        while(!queue.isEmpty()){ 
            int levelNum = queue.size();
            List<Integer> subList = new LinkedList<Integer>();
            
            for(int i=0; i<levelNum; i++) {
                if(queue.peek().left != null) 
                queue.offer(queue.peek().left);
                
                if(queue.peek().right != null) 
                queue.offer(queue.peek().right);
                
                subList.add(queue.poll().val);
            }
            wrapList.add(subList);
        }
        return wrapList;
    }
}