LeetCode OJ

这个题目我觉得是BFS的扩展，我只是对偶数层的节点先放到一个栈中，这样从栈出来后就换了一个顺序。

下面是AC代码：

/**
*Given a binary tree, return the zigzag level order traversal of its nodes' values.  *(ie, from left to right, then right to left for the next level and alternate between).
*
**/
public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root){
        ArrayList<ArrayList<Integer>> r = new ArrayList<ArrayList<Integer>>();
        if(root == null)
            return r;
        //for BFS
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        // to record the level information
        LinkedList<Integer> level = new LinkedList<Integer>();
        // to reverse the order 
        LinkedList<Integer> stack = null;
        ArrayList<Integer> sub = null;
        queue.offer(root);
        level.offer(1);
        int last = -1;
        while(!queue.isEmpty()){
            TreeNode cur = queue.poll();
            int la = level.poll();
            //135... directly put into an array
            if(la!=last && la%2 == 1){
                //put the last level into the total result;
                if(stack!=null){
                    ArrayList<Integer> rs = new ArrayList<Integer>();
                    while(!stack.isEmpty())
                        rs.add(stack.pop());
                    r.add(rs);
                }
                sub = new ArrayList<Integer>();
                sub.add( cur.val);
            }else if(la!=last && la%2 == 0){
                //put the last level into the total result
                if(sub!=null)
                    r.add(sub);
                stack = new LinkedList<Integer>();
                stack.push(cur.val);
            }else if(la == last && la%2 == 1){
                sub.add(cur.val);
            }else if(la == last && la%2 == 0){
                stack.push(cur.val);
            }
            if(cur.left != null){
                queue.offer(cur.left);
                level.offer(la+1);
            }
            if(cur.right != null){
                queue.offer(cur.right);
                level.offer(la+1);
            }    
            last = la;    
        }
        //put the last level into the total result
        if(last%2 == 1)
            r.add(sub);
        else{
            ArrayList<Integer> rs = new ArrayList<Integer>();
            while(!stack.isEmpty())
                rs.add(stack.pop());
            r.add(rs);
        }
        return r;
    }