題目：請實現一個函數，分別用來序列化和反序列化二叉樹
思路：序列化的時候，用中序遍歷，注意葉子節(jié)點的左右子節(jié)點用“#”代替；反序列化的時候，也是按照中序遍歷的順序。
解決方案：

public class Question37 {
    static class BinaryTreeNode {
        int value;
        BinaryTreeNode left;
        BinaryTreeNode right;

        public BinaryTreeNode(int value) {
            this.value = value;
        }
    }
    public static String serialize(BinaryTreeNode root){
        StringBuffer sb = new StringBuffer();
        if (root == null){
            sb.append("#,");
            return sb.toString();
        }
        sb.append(root.value + ",");
        sb.append(serialize(root.left));
        sb.append(serialize(root.right));
        return sb.toString();
    }
    static int index = -1;
    public static BinaryTreeNode deSerialize(String str){
        index++;
        int len = str.length();
        if (index >= len) return null;
        String[] strArray = str.split(",");
        BinaryTreeNode node = null;
        if (!strArray[index].equals("#")){
            node = new BinaryTreeNode(Integer.valueOf(strArray[index]));
            node.left = deSerialize(str);
            node.right = deSerialize(str);
        }
        return node;
    }

    public static void main(String[] args) {
        BinaryTreeNode pHead = new BinaryTreeNode(3);
        BinaryTreeNode pAhead = new BinaryTreeNode(2);
        BinaryTreeNode pBhead = new BinaryTreeNode(5);
        BinaryTreeNode pChead = new BinaryTreeNode(7);
        BinaryTreeNode pDhead = new BinaryTreeNode(1);
        pHead.left = pAhead;
        pHead.right = pBhead;
        pBhead.left = pChead;
        pAhead.left = pDhead;

        System.out.println(serialize(pHead));
    }
}