poj3626

bfs

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

#define maxx 1005
#define inc 502
#define maxn maxx * maxx

struct Point
{
 int x, y;
} q[maxx * maxx];

int n, cx, cy;
bool vis[maxx][maxx];
bool map[maxx][maxx];
int dist[maxx][maxx];
int dir[4][2] =
{
{ 0, 1 },
{ 1, 0 },
{ 0, -1 },
{ -1, 0 } };

bool ok(Point &a)
{
 if (a.x < 0 || a.y < 0 || a.x >= maxx || a.y >= maxx)
 return false;
 return !vis[a.x][a.y] && !map[a.x][a.y];
}

int bfs()
{
 int front = 0;
 int rear = 1;
 q[0].x = inc;
 q[0].y = inc;
 memset(vis, 0, sizeof(vis));
 vis[inc][inc] = true;
 dist[inc][inc] = 0;
 while (front != rear)
 {
 Point a = q[front++];
 if (front == maxn)
 front = 0;
 for (int i = 0; i < 4; i++)
 {
 Point b;
 b.x = a.x + dir[i][0];
 b.y = a.y + dir[i][1];
 if (ok(b))
 {
 q[rear++] = b;
 if (rear == maxn)
 rear = 0;
 dist[b.x][b.y] = dist[a.x][a.y] + 1;
 vis[b.x][b.y] = true;
 if (b.x == cx && b.y == cy)
 return dist[b.x][b.y];
 }
 }
 }
 return -1;
}

int main()
{
 //freopen("t.txt", "r", stdin);
 scanf("%d%d%d", &cx, &cy, &n);
 cx += inc;
 cy += inc;
 memset(map, 0, sizeof(map));
 for (int i = 0; i < n; i++)
 {
 int a, b;
 scanf("%d%d", &a, &b);
 a += inc;
 b += inc;
 map[a][b] = true;
 }
 int ans = bfs();
 printf("%d\n", ans);
 return 0;
}