257. 二叉树的所有路径

BFS解，时间O(n^2)，空间O(n^2)

    public List<String> binaryTreePaths(TreeNode root) {
        List<String> paths = new ArrayList<String>();
        if(root==null) return paths;
        Queue<TreeNode> queue1 = new LinkedList<TreeNode>();
        Queue<String> queue2 = new LinkedList<String>();
        queue1.add(root);
        queue2.add(Integer.toString(root.val));
        while(!queue1.isEmpty()){
            TreeNode temp = queue1.poll();
            String path = queue2.poll();            
            if(temp.left==null && temp.right==null){
                paths.add(path);
                continue;
            }
            if(temp.left!=null){
                queue1.add(temp.left);
                queue2.add(new StringBuilder(path).append("->"+temp.left.val).toString());
            }
            if(temp.right!=null){
                queue1.add(temp.right);
                queue2.add(new StringBuilder(path).append("->"+temp.right.val).toString());
            }
        }
        return paths;
    }