【ACM】蛇形填数

先判断，再移动，而不是发现越界了再退回来。 

#include "stdio.h"
#include "string.h"
#define maxn 20
int a[maxn][maxn];
int main()
{
	int n, x, y, tot = 0;
	scanf_s("%d", &n);
	memset(a, 0, sizeof(a));
	tot = a[x = 0][y = n - 1] = 1;
	while (tot < n*n)
	{
		while (x + 1 < n && !a[x + 1][y])	a[++x][y] = ++tot;
		while (y - 1 >= 0 && !a[x][y - 1])	a[x][--y] = ++tot;
		while (x - 1 >= 0 && !a[x - 1][y])	a[--x][y] = ++tot;
		while (y + 1 < n && !a[x][y + 1])	a[x][++y] = ++tot;
	}
	for (x = 0; x < n; x++)
	{
		for (y = 0; y < n; y++)
		{
			printf("%-3d ", a[x][y]);
		}
		printf("\n");
	}
	return 0;
}