Binary Tree Zigzag Level Order Traversal

BFS

public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        LinkedList<TreeNode> visiting = new LinkedList<TreeNode>();
        LinkedList<TreeNode> next = new LinkedList<TreeNode>();
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        
        if(root == null)
            return result;
        int level = 0;
        visiting.add(root);
        while(!visiting.isEmpty()){
            ArrayList<Integer> now = new ArrayList<Integer>();
            while(!visiting.isEmpty()){
                TreeNode tmp = visiting.poll();
                now.add(tmp.val);
                if(tmp.left!=null)
                    next.add(tmp.left);
                if(tmp.right!=null)
                    next.add(tmp.right);
            }
            
            if(level%2 == 1){
                int start = 0;
                int end = now.size()-1;
                while(start < end){
                    int tmp = now.get(start);
                    now.set(start, now.get(end));
                    now.set(end, tmp);
                    start++;
                    end--;
                }
            }
            
            result.add(now);
            visiting.addAll(next);
            next.clear();
            level++;
        }
        return result;
        
    }
}