Consider a permutation of the first n positive integers. for example, if n = 10, then a possible permutation is p = [5,2,1,8,4,7,10,9,3,6]. consider a street that is 10 blocks long, where there is a building of height p[i] on the ith block. for example, there is a building of height 5 on the first block, a building of height 2 on the second block, a building of height 1 on the third block, and so on. if you are standing at the very left end of the street, you will be able to see some of these 10 buildings, but unfortunately many of them will be blocked by a taller building.