103. Binary Tree Zigzag Level Order Traversal

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
if(root == null) {
return res;
}
LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
int lastLevelCnt = 1;
int curLevelCnt = 0;
List<Integer> curLevel = new ArrayList<Integer>();
boolean reverse = false;
while(!queue.isEmpty()) {
TreeNode cur = queue.poll();
curLevel.add(cur.val);
lastLevelCnt--;
if(cur.left != null) {
queue.offer(cur.left);
curLevelCnt++;
}
if(cur.right != null) {
queue.offer(cur.right);
curLevelCnt++;
}
if(lastLevelCnt == 0) {
if(reverse) {
Collections.reverse(curLevel);
}
reverse = !reverse;
res.add(curLevel);
lastLevelCnt = curLevelCnt;
curLevelCnt = 0;
curLevel = new ArrayList<Integer>();
}
}
return res;
}