103. Binary Tree Zigzag Level Order Traversal

class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> res=new ArrayList<List<Integer>>();
if(root==null)
return res;
Queue<TreeNode> queue=new LinkedList<TreeNode>();
List<Integer> list=new ArrayList<Integer>();
queue.add(root);
queue.add(null);
boolean zig=true;
while(!queue.isEmpty())
{
TreeNode node=queue.poll();
if(node==null)
{
res.add(new ArrayList<Integer>(list));
if(!queue.isEmpty())
queue.add(node);
list.clear();
zig=!zig;
}
else
{
if(zig==true)
list.add(node.val);
else
list.add(0, node.val);
if(node.left!=null)
queue.add(node.left);
if(node.right!=null)
queue.add(node.right);
}
}
return res;
}
}