Binary Tree Zigzag Level Order Traversal

public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
// Start typing your Java solution below
// DO NOT write main() function
ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
if(root == null) return result;
LinkedList<TreeNode> level = new LinkedList<TreeNode>();
level.add(root);
boolean flag = true;
while(!level.isEmpty()) {
int size = level.size();
ArrayList<Integer> subResult = new ArrayList<Integer>();
for(int i = 0; i < size; i++) {
TreeNode tmp = level.remove();
if(tmp.left != null) level.add(tmp.left);
if(tmp.right != null) level.add(tmp.right);
if(flag == true) subResult.add(tmp.val);
else subResult.add(0, tmp.val);
}
flag = (flag == true) ? false : true;
result.add(subResult);
}
return result;
}

public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
// Start typing your Java solution below
// DO NOT write main() function
ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
if(root == null)return result;
LinkedList<TreeNode> level = new LinkedList<TreeNode>();
level.add(root);
level.add(null);
boolean flag = true;
while(!level.isEmpty()) {
ArrayList<Integer> sub = new ArrayList<Integer>();
while(!level.isEmpty()) {
TreeNode tmp = level.remove();
if(tmp == null) {
if(!level.isEmpty()) level.add(null);
break;
}else {
if(flag == true) sub.add(tmp.val);
else sub.add(0, tmp.val);
if(tmp.left != null) level.add(tmp.left);
if(tmp.right != null) level.add(tmp.right);
}
}
result.add(sub);
flag = (flag == true) ? false : true;
}
return result;
}