BinaryTreeLevelOrderTraversal.java

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