B. Binary-tree-array:

Just convert the string into binary notation.
ie:L to 0 and R to 1.
let that value be b in decimal.
then the answer will be 2^n+b. where 2^n is the position of first node at height n, and b is distance between first node at height n and position of node having sequence of traversal(from root 1) as binary representation of b.
note: hidden node will be at height n.

code okay here...
int main(){
}