1. CS 3343: Analysis of Algorithms
Analyzing recursive algorithms
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2. Problem of the day
How do you find a coffee shop if you don’t know on which direction it might be?
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3. Analyzing recursive algorithms
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4. Recursive algorithms General idea: Divide and Conquer
Divide a large problem into smaller ones
By a constant ratio
By a constant or some variable
Solve each smaller one recursively or explicitly
Combine the solutions of smaller ones to form a solution for the original problem
Divide and Conquer
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5. Merge sort MERGE-SORT A[1 . . n] If n = 1, done.
Recursively sort A[ n/2 ] and A[ n/2 n ] .
“Merge” the 2 sorted lists.
Key subroutine: MERGE
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6. Merging two sorted arrays
Subarray 1
Subarray 2 20 13 7 2 12 11 9 1
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7. Merging two sorted arrays
Subarray 1
Subarray 2 20 13 7 2 12 11 9 1
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8. Merging two sorted arrays20 13 7 2 12 11 9 1
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9. Merging two sorted arrays20 13 7 2 12 11 9 1
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10. Merging two sorted arrays20 13 7 2 12 11 9 1 1
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11. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 1
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12. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 1 2
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13. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 20 13 7 12 11 9 1 2
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14. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 20 13 7 12 11 9 1 2 7
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15. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 20 13 7 12 11 9 20 13 12 11 9 1 2 7
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16. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 20 13 7 12 11 9 20 13 12 11 9 1 2 7 9
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17. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 20 13 7 12 11 9 20 13 12 11 9 20 13 12 11 1 2 7 9
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18. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 20 13 7 12 11 9 20 13 12 11 9 20 13 12 11 1 2 7 9 11
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19. Merging two sorted arrays20 13 7 2 12 11 9 1 20 13 7 2 12 11 9 20 13 7 12 11 9 20 13 12 11 9 20 13 12 11 20 13 12 1 2 7 9 11
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20. Merging two sorted arrays20 13 7 2 12 11 9 1
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21. How to show the correctness of a recursive algorithm?
By induction:
Base case: prove it works for small examples
Inductive hypothesis: assume the solution is correct for all sub-problems
Step: show that, if the inductive hypothesis is correct, then the algorithm is correct for the original problem.
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22. Correctness of merge sort
MERGE-SORT A[1 . . n]
If n = 1, done.
Recursively sort A[ n/2 ] and A[ n/2 n ] .
“Merge” the 2 sorted lists.
Proof:
Base case: if n = 1, the algorithm will return the correct answer because A[1..1] is already sorted.
Inductive hypothesis: assume that the algorithm correctly sorts smaller suarrays, i.e., A[1.. n/2 ] and A[n/2+1..n].
Step: if A[1.. n/2 ] and A[n/2+1..n] are both correctly sorted, the whole array A[1.. n] is sorted after merging.
Therefore, the algorithm is correct.
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23. How to analyze the time-efficiency of a recursive algorithm?
Express the running time on input of size n as a function of the running time on smaller problems
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24. Analyzing merge sort T(n) MERGE-SORT A[1 . . n] Θ(1) 2T(n/2)
f(n)
MERGE-SORT A[1 . . n]
If n = 1, done.
Recursively sort A[ n/2 ] and A[ n/2 n ] .
“Merge” the 2 sorted lists
Sloppiness: Should be T( n/2 ) + T( n/2 ) , but it turns out not to matter asymptotically.
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25. Analyzing merge sort T(n) = 2 T(n/2) + f(n) +Θ(1) Divide: Trivial.
Conquer: Recursively sort 2 subarrays.
Combine: Merge two sorted subarrays
T(n) = 2 T(n/2) + f(n) +Θ(1)
# subproblems
Dividing and
Combining
subproblem size
What is the time for the base case?
What is f(n)?
What is the growth order of T(n)?
Constant
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26. Merging two sorted arrays20 13 7 2 12 11 9 1
Θ(n) time to merge a total of n elements (linear time).
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27. Recurrence for merge sort
T(n) =
Θ(1) if n = 1;
2T(n/2) + Θ(n) if n > 1.
Later we shall often omit stating the base case when T(n) = Q(1) for sufficiently small n, but only when it has no effect on the asymptotic solution to the recurrence.
But what does T(n) solve to? I.e., is it O(n) or O(n2) or O(n3) or …?
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28. Binary Search To find an element in a sorted array, we
Check the middle element
If ==, we’ve found it
else if less than wanted, search right half
else search left half
Example: Find 9 3 5 7 8 9 12 15
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29. Binary Search To find an element in a sorted array, we
Check the middle element
If ==, we’ve found it
else if less than wanted, search right half
else search left half
Example: Find 9 3 5 7 8 9 12 15
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30. Binary Search To find an element in a sorted array, we
Check the middle element
If ==, we’ve found it
else if less than wanted, search right half
else search left half
Example: Find 9 3 5 7 8 9 12 15
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31. Binary Search To find an element in a sorted array, we
Check the middle element
If ==, we’ve found it
else if less than wanted, search right half
else search left half
Example: Find 9 3 5 7 8 9 12 15
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32. Binary Search To find an element in a sorted array, we
Check the middle element
If ==, we’ve found it
else if less than wanted, search right half
else search left half
Example: Find 9 3 5 7 8 9 12 15
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33. Binary Search To find an element in a sorted array, we
Check the middle element
If ==, we’ve found it
else if less than wanted, search right half
else search left half
Example: Find 9 3 5 7 8 9 12 15
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34. What’s the recurrence for the running time of BinarySearch?
BinarySearch (A[1..N], value) {
if (N == 0)
return -1; // not found
mid = (1+N)/2;
if (A[mid] == value)
return mid; // found
else if (A[mid] < value) return BinarySearch (A[mid+1, N], value) else
return BinarySearch (A[1..mid-1], value); }
What’s the recurrence for the running time of BinarySearch?
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35. Recurrence for binary search
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36. Recursive Insertion Sort
RecursiveInsertionSort(A[1..n])
1. if (n == 1) do nothing;
2. RecursiveInsertionSort(A[1..n-1]);
3. Find index i in A such that A[i] <= A[n] < A[i+1];
4. Insert A[n] after A[i];
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37. Recurrence for insertion sort
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38. Compute factorial Factorial (n) if (n == 1) return 1;
return n * Factorial (n-1);
Note: here we use n as the size of the input. However, usually for such algorithms we would use log(n), i.e., the bits needed to represent n, as the input size.
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39. Recurrence for computing factorial
Note: here we use n as the size of the input. However, usually for such algorithms we would use log(n), i.e., the bits needed to represent n, as the input size.
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40. What do these mean?
Challenge: how to solve the recurrence to get a closed form, e.g. T(n) = Θ (n2) or T(n) = Θ(nlgn), or at least some bound such as T(n) = O(n2)?
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41. Solving recurrence
Running time of many algorithms can be expressed in one of the following two recursive forms or
Both can be very hard to solve. We focus on relatively easy ones, which you will encounter frequently in many real algorithms (and exams…)
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42. Solving recurrence Recursion tree / iteration method
Substitution method
Master method
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