4.八皇后问题公式解
#include <stdio.h>
#include <stdlib.h>

#define MAXSIZE 30
#define MARK 'Q'

void initial(void);
void display(void);
void class_1(void);
void class_2(void);
void class_3(void);
void class_4(void);

int n; /* input board size */
char board[MAXSIZE+1][MAXSIZE+1]; /* the chess board */

void main(void)
{
void (*funct[])() = { class_1, class_2, /* functions */
class_4, class_3, /* of four */
class_1, class_2 /* classes */
};
char line[100];

printf("\nOne Solution for N Queens' Problem");
printf("\n==================================");
printf("\n\nBoard Size Please (N > 3) --> ");
gets(line);
n = atoi(line);

if (n > 3) {
initial(); /* initial the chess board */
(*funct[n % 6])(); /* call appropriate funct. */
display(); /* print result */
}
else
printf("\nIllegal Board Size.");
}

/* ------------------------------------------------------ */
/* FUNCTION initial : */
/* Initialize the chess board to blank. */
/* ------------------------------------------------------ */

void initial(void)
{
int i, j;

for (i = 1; i <= n; i++)
for (j = 1; j <= n; j++)
board[i][j] = ' ';
}

/* ------------------------------------------------------ */
/* FUNCTION display : */
/* Function to display the chess board. */
/* ------------------------------------------------------ */

#define DRAWGRID(N) { int i; \
printf("\n+"); \
for (i = 1; i <= N; i++) \
printf("-+"); \
}

#define DRAWLINE(N, r) { int i; \
printf("\n|"); \
for (i = 1; i <= N; i++) \
printf("%c|", board[r][i]);\
}

void display(void)
{
int r;

DRAWGRID(n);
for (r = 1; r <= n; r++) {
DRAWLINE(n, r);
DRAWGRID(n);
}
}

/* ------------------------------------------------------ */
/* FUNCTION class_1 : */
/* Solution for n mod 6 == 0 or 4. */
/* ------------------------------------------------------ */

void class_1(void)
{
int i;

for (i = 1; i <= n/2; i++)
board[2*i][i] = board[2*i-1][n/2+i] = MARK;
}

/* ------------------------------------------------------ */
/* FUNCTION class_2 : */
/* Solution for n mod 6 == 1 or 5. */
/* ------------------------------------------------------ */

void class_2(void)
{
int i;

for (i = 1; i <= (n-1)/2; i++)
board[2*i][i] = MARK;
for (i = 1; i <= (n+1)/2; i++)
board[2*i-1][(n-1)/2+i] = MARK;
}

/* ------------------------------------------------------ */
/* FUNCTION class_3 : */
/* Solution for n mod 6 == 3. */
/* ------------------------------------------------------ */

void class_3(void)
{
int i;

for (i = 1; i <= (n-3)/2; i++)
board[2*i+2][i] = board[2*i+3][(n-1)/2+i] = MARK;
board[1][n-1] = board[2][(n-2)/2] = board[3][n] = MARK;
}

/* ------------------------------------------------------ */
/* FUNCTION class_4 : */
/* Solution for n mod 6 = 2. */
/* ------------------------------------------------------ */

void class_4(void)
{
int i;

if (n > 8) {
for (i = 1; i <= 3; i++)
board[2*i-1][n/2-2+i] = MARK;
board[2][n] = board[4][1] = board[6][n-1] = MARK;
for (i = 1; i <= n/2 - 3; i++)
board[2*i+5][i+1] = board[2*i+6][n/2+1+i] = MARK;
}
else {
for (i = 1; i <= 4; i++)
board[2*i][i] = MARK;
board[1][6] = board[3][5] = MARK;
board[5][8] = board[7][7] = MARK;
}
}