Block Game

In the magic land, there is a smart but naughty boy Crane. Crane has n houses, and n friends (both numbered from 1 to n). Crane's mother has scheduled a plan for all his friends such that each friend visits each house of Crane exactly once at a different time, i.e., there is at most one friend in any house at any time. But like Crane all his friends are also naughty. Therefore his mother wants