[Luogu] 引水入城

https://www.luogu.org/problemnew/show/P1514

bfs + 线段覆盖

#include<bits/stdc++.h>

using namespace std;

struct sea {
    int h , num;
    bool got;
} s[];

bool cmp( sea a , sea b ) {
    return a.h > b.h;
}

const int dir[][] = { {  ,  } , {  , - } , {  ,  } , { - ,  } };
int ma[][] , n , m , ans , ct[];
bool built[];
queue<int> desert[];

void bfs( int start ) {
    bool went[][] = {  };
    queue<pair<int , int> > q;
    q.push( make_pair(  , start ) );
    if( n ==  ) {
        desert[start].push( start );
        ct[start]++;
    }
    while( !q.empty() ) {
        pair<int , int> a = q.front();
        q.pop();
        for( int i =  ; i <  ; i++ ) {
            pair<int , int> b = make_pair( a.first + dir[i][] , a.second + dir[i][] );
            if( b.first >=  && b.first < n && b.second >=  && b.second < m && went[b.first][b.second] ==  && ma[b.first][b.second] < ma[a.first][a.second] ) {
                q.push( b );
                if( b.first ==  )
                    s[b.second].got = ;
                if( b.first +  == n ) {
                    desert[b.second].push( start );
                    ct[start]++;
                }
                went[b.first][b.second] = ;
            }
        }
    }
}

void greedy() {
    int i = ;
    while( i < m ) {
        int maxnum =  , maxn;
        while( !desert[i].empty() ) {
            ct[desert[i].front()]--;
            if( ct[desert[i].front()] >= maxnum ) {
                maxnum = ct[desert[i].front()];
                maxn = desert[i].front();
            }
            desert[i].pop();
        }
        i++;
        while( ct[maxn] >  ) {
            while( !desert[i].empty() ) {
                ct[desert[i].front()]--;
                desert[i].pop();
            }
            i++;
        }
        ans++;
    }
}

int main() {
    cin >> n >> m;
    for( int i =  ; i < n ; i++ )
        for( int j =  ; j < m ; j++ )
            cin >> ma[i][j];
    for( int i =  ; i < m ; i++ )
        s[i].h = ma[][i] , s[i].num = i , s[i].got = ;
    sort( s , s + m , cmp );
    int p = ;
    while( p < m ) {
        if( !s[s[p].num].got )
            bfs( s[p].num );
        p++;
    }
    for( int i =  ; i < m ; i++ )
        if( desert[i].empty() )
            ans++;
    if( ans !=  ) {
        cout <<  << '\n' << ans;
        return ;
    }
    greedy();
    cout <<  << '\n' << ans;
}