1. Data Structures & Algorithms Recursion and Trees Richard Newman

2. Recursion Fundamental concept in math and CS Recursive definition Defined in terms of itself aN = a*aN-1, a0 = 1 Recursive function Calls itself int exp(int base, int pow) { return (pow == 0? 1 : base*exp(base, pow)); }

3. Recursion Recursive definition (and function) must: 1. have a base case – termination condition 2. always call a case smaller than itself All practical computations can be couched in a recursive framework! (see theory of computation)

4. Trees Recursively defined structures e.g., binary tree Base case: – Empty tree has no nodes Recursion: – None-empty tree has a root node with two children, each a tree

5. Roadmap to Recursion Widely used in CS and with trees...  Mathematical recurrences  Recursive programs  Divide and Conquer  Dynamic Programming  Tree traversal  DFS

6. Recursive Algorithms Recursive algorithm – solves problem by solving one or more smaller instances of same problem Recurrence relation – factorial – N! = N(N-1)!, for N > 0, with 0! = 1. – In C++, use recursive functions Int factorial(int N) { if (N == 0) return 1; return N*factorial(N-1); }

7. Recursive Algorithms BTW, can often also be expressed as iteration E.g., can also write N! computation as a loop: int factorial(int N) { for (int t = 1, i = 1; i <= N; ++i) t *= i; return t; }

8. Euclid's Algorithm Euclid's Algorithm is one of the oldest known algorithms Recursive method for finding the GCD of two integers int gcd(int m, int n) {// expect m >= n if (n == 0) return m; return gcd(n, m % n); } Base case Recursive call to smaller instance

9. Divide & Conquer Recursive scheme that divides input into two (or some fixed number) of (roughly) equal parts Then makes a recursive call on each part Widely used approach Many important algorithms Depending on expense of dividing and combining, can be very efficient

10. Divide & Conquer Example: find the maximum element in an array a[N] Easy to do iteratively... Base case: – Only one element – return it Divide: – Split array into upper and lower halves Recursion: – Find maximum of each half Combine results: – Return larger of two maxima

11. Divide & Conquer Property 5.1: A recursive function that divides a problem of size N into two independent (non-empty) parts that it solves, recursively calls itself less than N times. Prf: T(1) = 0 T(N) = T(k) + T(N-k) + 1 for recursive call on size N divided into one part of size k and the other of size N-k Induct!

12. Tower of Hanoi 3 pegs N disks, all on one peg Disks arranged from largest on bottom to smallest on top Must move all disks to target peg Can only move one disk at a time Must place disk on another peg Can never place larger disk on a smaller one Legend has it that the world will end when a certain group of monks finishes the task in a temple with 40 golden disks on 3 diamond pegs

13. Tower of Hanoi Target peg Which peg should top disk go on first?

14. Tower of Hanoi How many moves does this take?

15. Tower of Hanoi Property 5.2: The recursive d&c algorithm for the Towers of Hanoi problem produces a solution that has 2N – 1 moves. Prf: T(1) = 1 T(N) = T(N-1) + 1 + T(N-1) = 2 T(N-1) + 1 = 2N – 1 by induction

16. Divide & Conquer Two other important D&C algorithms: Binary search MergeSort Algorithm/metri c RecurrenceApprox. Soln. Binary Search comparisons C(N) = C(N/2)+1lg N MergeSort recursive calls A(N) = 2 A(N/2) + 1 N MergeSort comparisons C(N) = 2 C(N/2) + N N lg N

17. Dynamic Programming In Divide & Conquer, it is essential that the subproblems be independent (partition the input) When this is not the case, life gets complicated! Sometimes, we can essentially fill up a table with values we compute once, rather than recompute every time they are needed.

18. Dynamic Programming Fibonacci Numbers: F[0] = 0 F[1] = 1 F[N] = F[N-1] + F[N-2] Horribly inefficient implementation: int F(int N) { if (N < 1) return 0; if (N == 1) return 1; return F(N-1) + F(N-2); }

19. Dynamic Programming How bad is this code? How many calls does it make to itself? F(N) makes F(N+1) calls! Exponential!!!! 10 1 2 1 3 10 1 5 10 1 2 1 8 10 1 2 1 3 10 1 13 10 1 2 1 3 10 1 5 10 1 2 1

20. Dynamic Programming Can we do better? How? Make a table – compute once (yellow shapes) Fill up table 10 1 2 1 3 10 1 5 10 1 2 1 8 10 1 2 1 3 10 1 13 10 1 2 1 3 10 1 5 10 1 2 1

21. Dynamic Programming Property 5.3: Dynamic Programming reduces the running time of a recursive function to be at most the time it takes to evaluate the functions for all arguments less than or equal to the given argument, treating the cost of a recursive call as a constant.

22. Trees A mathematical abstraction Central to many algorithms Describe dynamic properties of algorithms Build and use explicit tree data structures Examples: Family tree of descendants Sports tournaments (Who's In?) Organization Charts (Army) Parse tree of natural language sentence File systems

23. Types of Trees Trees Rooted trees Ordered trees M-ary trees and binary trees Defn: A Tree is a nonempty collection of vertices and edges such that there is exactly one path between each pair of vertices. Defn: A path is a list of distinct vertices such that successive vertices have an edge between them Defn: A graph in which there is at most one path between each pair of vertices is a forest.