This solution to cs50's Game of Fifteen problem uses helper files supplied by the teaching staff

fifteen

/**
 * fifteen.c
 *
 * Computer Science 50
 * Problem Set 3
 *
 * Implements the Game of Fifteen (generalized to d x d).
 *
 * Usage: ./fifteen d
 *
 * whereby the board's dimensions are to be d x d,
 * where d must be in [MIN,MAX]
 *
 * Note that usleep is obsolete, but it offers more granularity than
 * sleep and is simpler to use than nanosleep; `man usleep` for more.
 */
 
#define _XOPEN_SOURCE 500

#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

// board's minimal dimension
#define MIN 3

// board's maximal dimension
#define MAX 9

// board, whereby board[i][j] represents row i and column j
int board[MAX][MAX];

// board's dimension
int d;

// prototypes
void clear(void);
void greet(void);
void init(void);
void draw(void);
bool move(int tile);
bool won(void);
void save(void);

int main(int argc, string argv[])
{
    // greet player
    greet();

    // ensure proper usage
    if (argc != 2)
    {
        printf("Usage: ./fifteen d\n");
        return 1;
    }

    // ensure valid dimensions
    d = atoi(argv[1]);
    if (d < MIN || d > MAX)
    {
        printf("Board must be between %i x %i and %i x %i, inclusive.\n",
            MIN, MIN, MAX, MAX);
        return 2;
    }

    // initialize the board
    init();

    // accept moves until game is won
    while (true)
    {
        // clear the screen
        clear();

        // draw the current state of the board
        draw();

        // saves the current state of the board (for testing)
        save();

        // check for win
        if (won())
        {
            printf("ftw!\n");
            break;
        }

        // prompt for move
        printf("Tile to move: ");
        int tile = GetInt();

        // move if possible, else report illegality
        if (!move(tile))
        {
            printf("\nIllegal move.\n");
            usleep(500000);
        }

        // sleep for animation's sake
        usleep(500000);
    }

    // that's all folks
    return 0;
}

/**
 * Clears screen using ANSI escape sequences.
 */
void clear(void)
{
    printf("\033[2J");
    printf("\033[%d;%dH", 0, 0);
}

/**
 * Greets player.
 */
void greet(void)
{
    clear();
    printf("GAME OF FIFTEEN\n");
    usleep(2000000);
}

/**
 * Initializes the game's board with tiles numbered 1 through d*d - 1,
 * (i.e., fills board with values but does not actually print them),
 * whereby board[i][j] represents row i and column j.
 */
void init(void)
{
    // define a counter and populate the array
    int c = 1;
    for (int i = 0; i < d; i++)
    {
        for (int j = 0; j < d; j++)
        {
        // fill the tiles in descending order
        board[i][j] = d * d - c;
        c++;
        }
    }
    //make the last tile blank
    board[d-1][d-1] = 99;
    // if there are an odd number of tiles, switch 1 and 2
    if ((d * d)%2 == 0)
    {
        board[d-1][d-2] = 2;
        board[d-1][d-3] = 1;
    }
}

/**
 * Prints the board in its current state.
 */
void draw(void)
{
    for (int i = 0; i < d; i++)
    {
        for (int j = 0; j < d; j++) 
            if (board[i][j] == 99)
            // print the blank tile
                printf(" _ ");
            else
            // the tiles in descending order
                printf("%2d ", board[i][j]);
        }
        printf("\n");
    }
}

/**
 * If tile borders empty space, moves tile and returns true, else
 * returns false. 
 */
bool move(int tile)
{
    // establish algorithm for checking bordering tiles
    for (int i = 0; i < d; i++)
    {
        for (int j = 0; j < d; j++)
        {
            if (board[i][j] == tile)
            {
                //check if the blank is to the right
                if (j+1 <= d-1 && board[i][j+1] == 99)
                {
                    board[i][j+1] = tile;
                    board[i][j] = 99;
                    return true;
                }
                // check if the blank is to the left
                else if (j-1 >= 0 && board[i][j-1] == 99)
                {
                    board[i][j-1] = tile;
                    board[i][j] = 99;
                    return true;
                }
                // check if the blank is above
                else if (i-1 >= 0 && board[i-1][j] == 99)
                {
                    board[i-1][j] = tile;
                    board[i][j] = 99;
                    return true;
                }
                // check if the blank is below
                else if (i+1 <= d-1 && board[i+1][j] == 99)
                {
                    board[i+1][j] = tile;
                    board[i][j] = 99;
                    return true;
                }
            
            }
        }
    }
    return false;
}

/**
 * Returns true if game is won (i.e., board is in winning configuration), 
 * else false.
 */
bool won(void)
{
    // check if board is sorted
    int n = 1;
    
    // check to see if last tile is blank and return false if it is not
    if (board[d-1][d-1] != 99)
        return false;
    
    // set up for loops to check if numbers are in ascending order beginning with 1
    for (int i = 0; i < d; i++)
    {
        for (int j = 0; j < d; j++)
        {
            // check last grid position first, if blank, return true
            if (i == d - 1 && j == d - 1)
                return true;
                
            // check the numbers on the rest of the tiles
            if (n != board[i][j])
                return false;
            n++;                        
        }
    }
    return false;
}

/**
 * Saves the current state of the board to disk (for testing).
 */
void save(void)
{
    // log
    const string log = "log.txt";

    // delete existing log, if any, before first save
    static bool saved = false;
    if (!saved)
    {
        unlink(log);
        saved = true;
    }

    // open log
    FILE* p = fopen(log, "a");
    if (p == NULL)
    {
        return;
    }

    // log board
    fprintf(p, "{");
    for (int i = 0; i < d; i++)
    {
        fprintf(p, "{");
        for (int j = 0; j < d; j++)
        {
            fprintf(p, "%i", board[i][j]);
            if (j < d - 1)
            {
                fprintf(p, ",");
            }
        }
        fprintf(p, "}");
        if (i < d - 1)
        {
            fprintf(p, ",");
        }
    }
    fprintf(p, "}\n");

    // close log
    fclose(p);
}

find

/**
 * find.c
 *
 * Computer Science 50
 * Problem Set 3
 *
 * Prompts user for as many as MAX values until EOF is reached, 
 * then proceeds to search that "haystack" of values for given needle.
 *
 * Usage: ./find needle
 *
 * where needle is the value to find in a haystack of values
 */
       
#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>

#include "helpers.h"

// maximum amount of hay
const int MAX = 65536;

int main(int argc, string argv[])
{
    // ensure proper usage
    if (argc != 2)
    {
        printf("Usage: ./find needle\n");
        return -1;
    }

    // remember needle
    int needle = atoi(argv[1]);

    // fill haystack
    int size;
    int haystack[MAX];
    for (size = 0; size < MAX; size++)
    {
        // wait for hay until EOF
        printf("\nhaystack[%d] = ", size);
        int straw = GetInt();
        if (straw == INT_MAX)
        {
            break;
        }
     
        // add hay to stack
        haystack[size] = straw;
    }
    printf("\n");

    // sort the haystack
    sort(haystack, size);

    // try to find needle in haystack
    if (search(needle, haystack, size))
    {
        printf("\nFound needle in haystack!\n\n");
        return 0;
    }
    else
    {
        printf("\nDidn't find needle in haystack.\n\n");
        return 1;
    }
}

generate

/**
 * generate.c
 *
 * Computer Science 50
 * Problem Set 3
 *
 * Generates pseudorandom numbers in [0,LIMIT), one per line.
 *
 * Usage: generate n [s]
 *
 * where n is number of pseudorandom numbers to print
 * and s is an optional seed
 */
       
#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define LIMIT 65536

int main(int argc, string argv[])
{
    // Make sure the user entered the proper command line arguments
    if (argc != 2 && argc != 3)
    {
        printf("Usage: ./generate n [s]\n");
        return 1;
    }

    // Convert the string to an integer
    int n = atoi(argv[1]);

    // Generate pseudorandom numbers using the seed, if provided,  or no seed if not
    if (argc == 3)
    {
        srand((unsigned int) atoi(argv[2]));
    }
    else
    {
        srand((unsigned int) time(NULL));
    }

    // Print the requested number of pseudorandom numbers
    for (int i = 0; i < n; i++)
    {
        printf("%i\n", rand() % LIMIT);
    }

    // that's all folks
    return 0;
}

helpers

/**
 * helpers.c
 *
 * Computer Science 50
 * Problem Set 3
 *
 * Helper functions for Problem Set 3.
 */
       
#include <cs50.h>

#include "helpers.h"

/**
 * Returns true if value is in array of n values, else false.
 */
bool search(int value, int values[], int n)
{
    // implement a binary search program
    // set the bounds of the list
    int beginning = 0;
    int end = n-1;
    
    // while the length of the list is more than zero
    while (beginning <= end)
    {
        // define the middle of the list
        int middle = (beginning + end) / 2;
        
        // if the middle number is the sought for number, return if true
        if (values[middle] == value)
        {
            return true;
        }
        
        // if the middle number is larger than the sought for number, search the left side of the list
        else if (values[middle] > value)
        {
            end = middle - 1;
        }
        // if the middle number is less than value, search to the right
        else
        {
            beginning = middle + 1;
        }
    }
    return false;
}

/**
 * Sorts array of n values.
 */
void sort(int values[], int n)
{
    // run the sort as many times as there are numbers
    for (int i = 0; i < n; i++)
    {
        // commit the smallest number to memory
        int smallest = i;
        
        // compare the smallest to the rest of the list
        for (int j = i + 1; j < n; j++)
        {
            // determine which number is smallest
            if (values[j] < values[smallest])
            {
                smallest = j;
            }
        }
        
        // save the smallest number and put the first number in the list in its place
        int tmp = values[smallest];
        values[smallest] = values[i];
        
        // place the smallest number at the beginning of the list
        values[i] = tmp;
    }
}