双向广搜WA一个点求助

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <queue>
#include <cstring>
using namespace std;
const int N = 1500;
char a[N][N];
int n;
int sx,sy,ex,ey;
queue <pair<int,int> > start;
queue <pair<int,int> > final;
int steps[N][N];
int stepe[N][N];
const int dx[4] = {-1,1,0,0};
const int dy[4] = {0,0,-1,1};
int bfs()
{
    start.push(make_pair(sx,sy));
    final.push(make_pair(ex,ey));
    steps[sx][sy] = 0;
    stepe[ex][ey] = 0;
    if(sx==ex&&sy==ey)
    {
        return 0;
    }
    while(!start.empty()||!final.empty())
    {
        int lens = start.size();
        int lene = final.size();
        // 正向
        for(register int i=0;i<lens;i++)
        {
            int curx = start.front().first, cury = start.front().second;
            start.pop();
            for(register int j=0;j<4;j++)
            {
                int xx = curx+dx[j];
                int yy = cury+dy[j];
                if(a[xx][yy] == '0' &&xx<=n&&yy<=n&&xx>=1&&yy>=1&&steps[xx][yy]==-1)
                {
                    if(stepe[xx][yy]!=-1)
                    {
                        return steps[curx][cury] + 1 + stepe[xx][yy];  //steps[curx][cury] + 1其实就是 steps[xx][yy]
                    }
                    steps[xx][yy] = steps[curx][cury] + 1;
                    start.push(make_pair(xx,yy));
                }
            }
        }
        //反向
        for(register int i= 0;i<lene;i++)
        {
            int curx = final.front().first, cury = final.front().second;
            final.pop();
            for(int j=0;j<4;j++)
            {
                int xx = curx + dx[j];
                int yy = cury + dy[j];
                if(a[xx][yy] == '0'&&xx<=n&&yy<=n&&xx>=1&&yy>=1&&stepe[xx][yy] == -1)
                {
                    if(steps[xx][yy]!=-1)
                    {
                        return steps[xx][yy] + stepe[curx][cury] + 1;
                    }
                    stepe[xx][yy] = stepe[curx][cury] + 1;
                    final.push(make_pair(xx,yy));
                }
            }
        }

    }
}
int main()
{
    memset(steps,-1,sizeof(steps));
    memset(stepe,-1,sizeof(stepe));
    scanf("%d",&n);
    for(register int i=1;i<=n;i++)
    {
        scanf("%s",&a[i]);
    }
    scanf("%d%d%d%d",&sx,&sy,&ex,&ey);
    printf("%d\n",bfs());
    return 0;
}