1. Lecture 30
CSE 331
Nov 10, 2017

2. Mini project video due Mon

3. Homework 8

4. Solutions to Homework 7
At the END of the lecture

5. Counting Inversions Input: n distinct numbers a1,a2,…,an
Inversion: (i,j) with i < j s.t. ai > aj
Output: Number of inversions

6. Divide and Conquer
Divide up the problem into at least two sub-problems
Solve all sub-problems: Mergesort
Recursively solve the sub-problems
Solve some sub-problems: Multiplication
Solve stronger sub-problems: Inversions
“Patch up” the solutions to the sub-problems for the final solution

7. Handling crossing inversionsaL aR 1
100 2 4
Why should aL and aR be sorted?
Sort aL and aR recursively!

8. Mergesort-Count algorithm
Input: a1, a2, …, an
Output: Numbers in sorted order+ #inversion
T(2) = c
T(n) = 2T(n/2) + cn
MergeSortCount( a, n )
If n = 1 return ( 0 , a1)
If n = 2 return ( a1 > a2, min(a1,a2); max(a1,a2))
O(n log n) time
aL = a1,…, an/2
aR = an/2+1,…, an
(cL, aL) = MergeSortCount(aL, n/2)
(cR, aR) = MergeSortCount(aR, n/2)
O(n)
Counts #crossing-inversions+ MERGE
(c, a) = MERGE-COUNT(aL,aR)
return (c+cL+cR,a)

9. Closest pairs of points
Input: n 2-D points P = {p1,…,pn}; pi=(xi,yi)
d(pi,pj) = ( (xi-xj)2+(yi-yj)2)1/2
Output: Points p and q that are closest

10. Group Talk time
O(n2) time algorithm?
1-D problem in time O(n log n) ?

11. Sorting to rescue in 2-D?
Pick pairs of points closest in x co-ordinate
Pick pairs of points closest in y co-ordinate
Choose the better of the two

12. A property of Euclidean distanceyj
d(pi,pj) = ( (xi-xj)2+(yi-yj)2)1/2 yi xi xj
The distance is larger than the x or y-coord difference

13. Rest of Today’s agenda
Divide and Conquer based algorithm

14. Dividing up P Q R
First n/2 points according to the x-coord

15. Recursively find closest pairsQ R
δ = min (blue, green)

16. An aside: maintain sorted lists
Px and Py are P sorted by x-coord and y-coord
Qx, Qy, Rx, Ry can be computed from Px and Py in O(n) time

17. An easy case R > δ Q All “crossing” pairs have distance > δ
δ = min (blue, green)

18. Life is not so easy thoughQ R
δ = min (blue, green)

19. Rest of Today’s agenda
Divide and Conquer based algorithm

20. Euclid to the rescue (?) d(pi,pj) = ( (xi-xj)2+(yi-yj)2)1/2 yj yi xi
The distance is larger than the x or y-coord difference

21. Life is not so easy thoughδ Q R
> δ
> δ
> δ
δ = min (blue, green)

22. All we have to do now δ R Q S
Figure if a pair in S has distance < δ S
δ = min (blue, green)

23. Assume can be done in O(n)
The algorithm so far…
O(n log n) + T(n)
Input: n 2-D points P = {p1,…,pn}; pi=(xi,yi)
Sort P to get Px and Py
O(n log n)
T(< 4) = c
Closest-Pair (Px, Py)
T(n) = 2T(n/2) + cn
If n < 4 then find closest point by brute-force
Q is first half of Px and R is the rest
O(n)
Compute Qx, Qy, Rx and Ry
O(n)
(q0,q1) = Closest-Pair (Qx, Qy)
O(n log n) overall
(r0,r1) = Closest-Pair (Rx, Ry)
O(n)
δ = min ( d(q0,q1), d(r0,r1) )
O(n)
S = points (x,y) in P s.t. |x – x*| < δ
return Closest-in-box (S, (q0,q1), (r0,r1))
Assume can be done in O(n)