Recursive : Divide and Conquer

Consider your input instance

• Allocate work
• construct one or more sub-instances

　　　　　1 assume by magic your friends give you the answer for these, trust your friends to sovle sub problem.

　　　　　2 each sub-instance must be a smaller instance of the same problem.

• Use this help to solve your own instance.

 

Do NOT worry about anything else.

• Who your boss is...
• How your friends solve their instances...
• No global variables or effects.

Recursive algorithm

• assume you have an algorithm that works
• use it to write an algorithm that works.

Recursive sorts

• given list of objects to be sorted
• split the list into two sub-lists
• recursively have a friend sort the two sub-lists.
• combine the two sorted-list into one entirely sorted list.

	size of sublist
	n/2,n/2	n-1,1
minimal efforts splitting, lots of efforts recombining.	merge sort	insertion sort
lots of efforts splitting, minimal efforts recombining.	quick sort	selection sort

partition sort: partition set into two using randomly chosen pivot.