17. (b) 6%

a = n1
b = n2
while a != b:
    a = a.p if a else n2
    b = b.p if b else n1
return a

In the above pseudo code, n1 and n2 are two nodes in a binary search tree $T$. a = a.p if a else n2 means if a is not NULL then set a to a.p otherwise set a to n2. What does this piece of code compute (what is "a" returned by the function)?