Count pairs from two BSTs whose sum is equal to a given value x

class Solution:
    def countPairs(self, root1, root2, x):
        cur1,cur2=root1,root2
        st1,st2=deque(),deque()
        count=0
        while True:
            while cur1:
                st1.append(cur1)
                cur1=cur1.left
            while cur2:
                st2.append(cur2)
                cur2=cur2.right
            if not st1 or not st2:
                break
            top1,top2=st1[-1],st2[-1]
            sum=top1.data+top2.data
            if sum<x:
                st1.pop()
                cur1=top1.right
            elif sum>x:
                st2.pop()
                cur2=top2.left
            else:
                count+=1
                st1.pop()
                st2.pop()
                cur1,cur2=top1.right,top2.left
        return count