Snake game DP problem given in some placement test I did for a friend XD

snake_dp.cpp

#include <iostream>
#include <algorithm>
#define INF 1000000007
using namespace std;

int dp[505][505][2];    // the storage for the DP (Dynamic programming)
int grid[505][505];     // the storage for the actual grid

void init(int n, int m){                                // read from input and initialize the grid and
    for(int i=0; i<n; i++){                             // the DP arrays.
        for(int j=0; j<m; j++){
            cin >> grid[i][j];
            if(grid[i][j] == -1) grid[i][j] = -INF;
            if(j==0){ 
                dp[i][j][0] = grid[i][j];
                dp[i][j][1] = grid[i][j];
            }
            else{
                dp[i][j][0] = -INF;
                dp[i][j][1] = -INF;
            }
        }
    }
}


int main(){
    int n, m, val;
    cin >> n >> m;  // read n and m
    init(n, m);     // the grid is read inside the init function

    // Do the DP now
    for(int j=0; j<m; j++){
        for(int i=n-1; i>0; i--)
            if(grid[i-1][j] > -1) dp[i-1][j][1] = max(dp[i-1][j][1], dp[i][j][1]+grid[i-1][j]);
        if(dp[0][j][1] > -1 && grid[n-1][j] > -1) dp[n-1][j][1] = max(dp[n-1][j][1], grid[n-1][j]);

        for(int i=0; i<n-1; i++)
            if(grid[i+1][j] > -1) dp[i+1][j][0] = max(dp[i+1][j][0], dp[i][j][0]+grid[i+1][j]);
        if(grid[0][j] > -1 && dp[n-1][j][0] > -1) dp[0][j][0] = max(dp[0][j][0], grid[0][j]);
        
        for(int i=0; i<n; i++){
            if(j+1 < m && grid[i][j+1] > -1){
                dp[i][j+1][0] = max(dp[i][j+1][0], max(dp[i][j][0], dp[i][j][1])+grid[i][j+1]);
                dp[i][j+1][1] = max(dp[i][j+1][1], max(dp[i][j][0], dp[i][j][1])+grid[i][j+1]);
            }
        }
    }

    // DP done. Now just read the results from the DP array and print the maximum value.
    int res = -1;
    for(int i=0; i<n; i++) res = max(res, max(dp[i][m-1][0], dp[i][m-1][1]));
    cout << res << endl;

    return 0;
}