Binary Tree Level Order Traversal

BTLOT.java

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