Luogu P1902 刺杀大使

#include<iostream>
#include<iomanip>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<queue>
using namespace std;
#define INF 0x7fffffff
inline int read() {
    char ch;
    bool bj=0;
    while(!isdigit(ch=getchar()))
        bj|=(ch==‘-‘);
    int res=ch^(3<<4);
    while(isdigit(ch=getchar()))
        res=(res<<1)+(res<<3)+(ch^(3<<4));
    return bj?-res:res;
}
void printnum(int x) {
    if(x>9)printnum(x/10);
    putchar(x%10+‘0‘);
}
inline void print(int x,char ch) {
    if(x<0) {
        putchar(‘-‘);
        x=-x;
    }
    printnum(x);
    putchar(ch);
}
int n,m,a[1005][1005],cnt,maxn;
int ans;
queue<pair<int,int> >q;
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
bool vst[1005][1005];
inline bool BFS(int limit){
    while(!q.empty())q.pop();
    memset(vst,0,sizeof(vst));
    q.push(make_pair(1,1));
    vst[1][1]=1;
    while(!q.empty()){
        int x=q.front().first,y=q.front().second;
        for(int i=0;i<4;i++){
            int tx=x+dx[i],ty=y+dy[i];
            if(tx>=1&&tx<=n&&ty>=1&&ty<=m&&a[tx][ty]<=limit&&!vst[tx][ty]){
                vst[tx][ty]=1;
                if(tx==n)return 1;
                q.push(make_pair(tx,ty));
            }
        }
        q.pop();
    }
    return 0;
}
signed main() {
    n=read();
    m=read();
    for(int i=1; i<=n; i++)
        for(int j=1; j<=m; j++){
            a[i][j]=read();
            maxn=max(maxn,a[i][j]);
        }
    int l=0,r=maxn;
    while(l<=r) {
        int mid=(l+r)>>1;
        if(BFS(mid)) {
            ans=mid;
            r=mid-1;
        }
        else l=mid+1;
    }
    print(ans,‘\n‘);
    return 0;
}