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Recursion: insert at tail, delete at tail and print list

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• Show off recursion
  • In this web page, I just want to show off how easy it is to use recursion to manipulate the tail element of a list

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  • I am applying this recursive solution technique:

    if ( problem is easy ) return solution else { have someone else some a smaller problem use the solution to solve MY problem }

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• Insert at tail
  • C code:

    struct ListElem *insertTail(struct ListElem *h, struct ListElem *e) { struct ListElem *sol; // The solution returned by my helper struct ListElem *mySol; // My solution if ( h == NULL ) { /* EASY case: list contains only element e */ e->next = NULL; return(e); } else { /* --------------------------------------------- DIFFICULT case: 1. give someone a smaller problem and 2. use its sol to solve my problem ---------------------------------------------- */ sol = insertTail( h->next, e); // (1) Have someone solve smaller prob // (Insert e in shorter list) h->next = sol; // (2) use sol to solve my problem // (Add sol after the first elem) return(h); // Claim glory... } }

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• Delete at tail
  • C code:

    struct ListElem *deleteTail ( struct ListElem *h ) { struct ListElem *sol; // The solution returned by my helper if ( h == NULL ) { /* EASY case: list is empty, nothing to delete */ return(NULL); // Return empty list } else if ( h->next == NULL ) { /* EASY case: (last element !) after deleting, list is empty */ free(h); // De-allocate !!! return(NULL); // Return empty list } else { /* --------------------------------------------- DIFFICULT case: 1. give someone a smaller problem and 2. use its sol to solve my problem ---------------------------------------------- */ sol = deleteTail( h->next ); // (1) Have someone solve smaller prob // (Delete tail in shorter list) h->next = sol; // (2) use sol to solve my problem // (Add sol after the first elem) return(h); // Claim glory... } }

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• Print a list (using recursion)
  • C code:

    void printList( struct ListElem *h ) { if ( h == NULL ) { /* EASY case: nothing to print ! */ return; } else { /* --------------------------------------------- DIFFICULT case: 1. give someone a smaller problem and 2. use its sol to solve my problem ---------------------------------------------- */ printf("%d ", h->value); // I print the head elem out first printList(h->next); // Have someone print the rest } }

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• Demo

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc recurse1.c To run:          ./a.out