You are given N blocks of height 1…N. In how many ways
can you arrange these blocks in a row such that when viewed from left
you see only L blocks (rest are hidden by taller blocks) and when seen
from right you see only R blocks? Example given N=3, L=2, R=1 there is
only one arrangement {2, 1, 3} while for N=3, L=2, R=2 there are two
ways {1, 3, 2} and {2, 3, 1}.

**Solutions:**