ztree树只允许兄弟节点之间拖拽怎么实现？

//zTree配置
var setting = {
    edit: {
        enable: true,
        showRemoveBtn: false,
        showRenameBtn: false
    },
    data: {
        simpleData: {
            enable: true
        }
    },
    callback: {
        beforeDrop: beforeDrop //拖拽操作结束之前的事件回调，该处用于判断是否兄弟节点间拖动
    }
};

//仅亲兄弟节点间可拖动
function beforeDrop(treeId, treeNodes, targetNode, moveType) {
    var moveNode = treeNodes[0];
    switch (moveType)
    {
        case "prev":
        case "next":
            if (moveNode.pId == targetNode.pId) {
                return true;
            }
            break;
        case "inner":
            if (moveNode.pId == targetNode.pId) {
                return true;
            }
            break;
    }
    return false;
}

//所有同层级节点间可拖动
function beforeDrop(treeId, treeNodes, targetNode, moveType) {
    var moveNode = treeNodes[0];
    switch (moveType) {
        case "prev":
        case "next":
            if (moveNode.level == targetNode.level) {
                return true;
            }
            break;
        case "inner":
            if (moveNode.level == targetNode.level + 1) {
                return true;
            }
            break;
    }
    return false;
}