LECTURE 21: RECURSION & LINKED LIST REVIEW CSC 212 – Data Structures

 Easy to create & use  Java support built-in  Immediate access  Can get any entry  Size is fixed from start  Recursive structure  Need DNode & Dlist  Slower to access  Usually need traversal  Size changes w/ data ArraysLinked Lists Linked Lists v. Arrays

 Linked lists not always obvious to code  Lots of references must be set (“aliased”) ‏  Easy to screw-up, especially by mixing ordering  Very easy solution to help do this, however  Begin by drawing picture of method does  Write method skeleton (declaration & braces {} ) ‏  Start method by aliasing variable to each instance  Now assign fields making them equal to your picture  Double-check by comparing trace with your picture Using Linked Lists

 Identify two parts of all recursive solutions  Cases when problem is trivial to solve “base case(s)”  Split into identical, smaller problems at RECURSIVE step(s) ‏  When both parts cannot be determined, stop  Do not always have recursive solution for a problem  Make sure you will always reach base case  Infinite recursion occurs when no base case reached… Recursive Solutions

Infinite Recursion

 Handles when only 1 possible solution remains  0 or 1 array entries remain to be processed  Linked list node’s next null or trailer (sentinel) ‏  At the last value to be processed by the method Writing Base Case

 Performs simple operation with 1 or 2 data items  Before recursive call(s) could perform the operation…  … or the recursive call(s) provide (some of) the data  Ultimately solves problem for data it considers  Mostly, uses recursive calls to do the work  Something is wrong if always solving entire problem  Question becomes: WHAT DATA TO CONSIDER?  Needs to shrink with each recursive call  Should be defined by method’s parameters Writing Recursive Step(s)‏

 Public method only has required parameters  Linked list instance to be processed  Reference to the array to be evaluated  Important value whose result the user is interested  Private method may have more parameters  Value or array index where method starts working  Value or array index where method stop working  Range or indices where method starts & stops Which Case is Which?

public int findMin(int[] arr) { return findMin(arr, 0); } private int findMin(int[] arr, int loc) { if (loc == arr.length – 1) { return arr[loc]; } else { int minAfter = findMin(arr, loc + 1); if (minAfter < arr[loc]) { return minAfter; } else { return arr[loc]; } } Where Method Starts Work

public void printTri(int[] arr) { return printTri(arr, arr.length - 1); } private int printTri(int[] arr, int loc) { if (loc == 0) { System.out.println(arr[loc]); } else { for (int i = 0; i < loc; i++) { System.out.print(arr[i] + “ ”); } System.out.println(); printTri(arr, loc - 1); } } Where Method Stops Work

public boolean pal(String str) { return pal(str, 0, str.length() - 1); } private boolean pal(String str, int start, int end){ if (start > end) { return true; } else { boolean plndrm = pal(str, start + 1, end - 1); char fst = str.charAt(start); char lst = str.charAt(end); return plndrm && (fst == lst); } } Range To Use

 Start week #8 assignment  Due by 5PM next Tuesday  Start programming assignment #3  Messages are not always sent to everyone!  Read sections in book before class  Is all code really equal?  How can we determine how fast code runs? Before Next Lecture… NO!