八皇后问题-递归与非递归方法实现

int board[8][8];
int cnt = 0;

bool isValid(int i, int j)
{
int k;
for(k = 0; k < 8; ++k)
if(k != j && board[i][k]) return false;
for(k = 0; k < 8; ++k)
if(k != i && board[k][j]) return false;
for(k = 1; k < 8 && i-k >= 0 && j-k >= 0; ++k)
if(board[i-k][j-k]) return false;
for(k = 1; k < 8 && i-k >= 0 && j+k < 8; ++k)
if(board[i-k][j+k]) return false;
return true;
}

void queen_rec(int row)
{
if(row == 8)
{
for(int i = 0; i < 8; ++i)
for(int j = 0; j < 8; ++j)
if(board[i][j]) printf("%d %d\n", i, j);
printf("\n"); ++cnt;
}
else
{
for(int i = 0; i < 8; ++i)
{
if(isValid(row, i))
{
board[row][i] = 1;
queen_rec(row+1);
board[row][i] = 0;
}
}
}
}

void queen_nonrec()
{
//use stack to hold the previous valid position of each row
int stack[8], row;
for(row = 0; row < 8; ++row) stack[row] = -1;
row = 0;
//the row variable will decrease by 1 whenever backtracking happens.
//Finally, the row variable will become -1 and the loop should end there.
while(row >= 0)
{
if(row == 8)
{
for(int i = 0; i < 8; ++i)
{
for(int j = 0; j < 8; ++j)
if(board[i][j]) printf("%2d",1);
else printf("%2d", 0);
printf("\n");
}
printf("\n"); ++cnt;
--row;
}
else
{
//recover current position to be valid
if(stack[row] >= 0 && stack[row] < 8) board[row][stack[row]] = 0;
//try to find the next position available
++stack[row];
while(stack[row] < 8 && !isValid(row, stack[row])) ++stack[row];
//when come to the end of current row, set the position index of current row back to -1 and go to the upper row
if(stack[row] >= 8) {stack[row] = -1; --row;}
else
{
//found a valid position, go to the next row
board[row][stack[row]] = 1; ++row;
}
}
}
}