Little Johnny is extremely fond of watching television. His parents are off for work for the period [S, F), and he wants to make full use of this time by watching as much television as possible: in fact, he wants to watch TV non-stop the entire period [S, F). He has a list of his favorite n TV shows (on different channels), where the i-th show runs for the time period [sifi), so that the union of [si,fi) fully covers the entire time period IS, F) when his parents are away. Little Johnny doesn't mind switching in the middle of a show he is watching, but is very lazy to switch TV channels, so he wants to find the smallest set of TV shows that he can watch, and still stay occupied for the entire period [S, F). Design an efficient O(n logn) greedy algorithm to help Little Johny