我在將 Treapnode 插入 Treap 時遇到問題。它接受 3 個參數。添加（E 鍵，P 優先級，treapnode x）。我嘗試了很多方法，但總是出現空指針異常。我嘗試檢查左右樹中的空情況。private TreapNode add (E key, P priority, TreapNode x)        throws ElementFoundException {    // For You To Complete    int compare = key.compareTo(x.element());    if (x == null){        return new TreapNode(key, priority);    }    //root is larger than the key    else if (compare == 0) {        throw new ElementFoundException("Element was found, and tree was not changed.");    } else if (compare < 0) {        if (x.left() == null) {         //TreapNode y = new TreapNode(key, priority);            TreapNode y = x.left;            x.left = y.right;            y.right = x;            return y;        } else {            x.left = add(key, priority, x.left());        }    }    //root is smaller than the key    else if (compare > 0) {        if (x.right() == null) {            //TreapNode y = new TreapNode(key, priority);            TreapNode z = x.right;            x.right = z.left;            z.left = x;            return z;        }    }    return x;}