LC 1349. Maximum Students Taking Exam

Posted 2021-03-11 feiief

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Solution 0:

class Solution {
public:
    int m,n;
    int maxStudents(vector<vector<char>>& seats) {
        m=seats.size();
        n=seats[0].size();
        vector<vector<int>> dp(m*n+2,vector<int>(1<<(2*n),-1));
        return dfs(seats,0,0,dp);
    }
    
    int dfs(vector<vector<char>>& seats, int pos, int state, vector<vector<int>> &dp){
        if(dp[pos][state]!=-1) return dp[pos][state];
        if(pos==m*n) return 0;
        int x=pos/n;
        int y=pos%n;
        if(y==0){
            state=state>>n;
        }
        int take=0,untake=0;
        untake=dfs(seats,pos+1,state,dp);
        int flag=0;
        if(seats[x][y]==‘#‘) flag=1;
        if(y-1>=0 && (state&(1<<(y-1+n)))>0) flag=1;
        if(y+1<n && (state&(1<<(y+1+n)))>0) flag=1;
        if(y-1>=0 && x>0 && (state&(1<<(y-1)))>0) flag=1;
        if(y+1<n && x>0 && (state&(1<<(y+1)))>0) flag=1;
        if(flag==0) take=1+dfs(seats,pos+1,state|(1<<(y+n)),dp);
        return dp[pos][state]=max(take,untake);
    }
};

 

 

 Solution 1 :

class Solution {
public:
    int m,n;
    int maxStudents(vector<vector<char>>& seats) {
        m=seats.size();
        n=seats[0].size();
        vector<vector<int>> dp(m+1,vector<int>(1<<(2*n),-1));
        return dfs(seats,0,0,dp);
    }
    
    int dfs(vector<vector<char>>& seats, int row, int state, vector<vector<int>> &dp){
        if(row==m) return 0;
        if(dp[row][state]!=-1) return dp[row][state];
        int ns=state>>n;  // when we want to skip to next row, need to change the state
        int tmp1= dfs(seats,row+1,ns,dp); 
        int tmp2=0;
        for(int i=0;i<n;++i){    // check whether we can fill seats[row][i]
            if(seats[row][i]==‘#‘) continue;
            if((state&(1<<(n+i)))>0) continue;
            if(i-1>=0 && (state&(1<<(n+i-1)))>0) continue; // left
            if(i+1<n && (state&(1<<(n+i+1)))>0) continue;  // right
            if(row>0 && i-1>=0 && (state&(1<<(i-1)))>0) continue; //topleft
            if(row>0 && i+1<n && (state&(1<<(i+1)))>0) continue; //topright
            tmp2=max(tmp2,1+dfs(seats,row,state|(1<<i+n),dp));
        }
        return dp[row][state]=max(tmp1,tmp2);
        
    }
};

 

Solution 2:

class Solution {
public:
    int m,n;
    
    int maxStudents(vector<vector<char>>& seats) {
        m=seats.size();
        n=seats[0].size();
        vector<vector<int>> dp(m+1,vector<int>(1<<n,-1));
        return dfs(seats,0,0,dp);
    }
    
    int dfs(vector<vector<char>>& seats, int row, int prestate, vector<vector<int>> &dp){
        if(row==m) return 0;
        if(dp[row][prestate]!=-1) return dp[row][prestate];
        
        vector<int> curstates;
        dfs1(seats[row],0,prestate,curstates,0);
        int res=0;
        for(int sta:curstates){
            int cnt=getcnt(sta);
            res=max(res,cnt+dfs(seats,row+1,sta,dp));
        }
        return dp[row][prestate]=res;
    }
    
    int getcnt(int sta){
        int res=0;
        for(int i=0;i<n;++i){
            if((sta&(1<<i))>0) res++;
        }
        return res;
    }
    
    void dfs1(vector<char> seat, int col, int prestate, vector<int>& curstates, int curstate){
        if(col==n) {
            curstates.push_back(curstate);
            return;
        }
        dfs1(seat,col+1,prestate,curstates,curstate);
        
        int flag=0;
        if(seat[col]==‘#‘) return;
        if(col-1>=0 && (((1<<(col-1))&curstate)>0 || ((1<<(col-1))&prestate)>0)) return;
        if(col+1<n && ((1<<(col+1))&prestate)>0) return;
        dfs1(seat,col+1,prestate,curstates,curstate|(1<<col));
    }
};