As Far from Land as Possible Solutions in C++

/// Source : https://leetcode.com/problems/as-far-from-land-as-possible/
/// Author : liuyubobobo
/// Time   : 2019-08-17

#include <iostream>
#include <vector>
#include <queue>

using namespace std;


/// BFS
/// Time Complexity: O(n * m)
/// Space Complexity: O(n * m)
class Solution {

private:
    int n, m;
    int dirs[4][2] = {{-1, 0}, {0, 1}, {1, 0}, {0, -1}};

public:
    int maxDistance(vector<vector<int>>& grid) {

        n = grid.size(), m = grid[0].size();
        vector<vector<int>> dis(n, vector<int>(m, -1));
        queue<int> q;
        for(int i = 0; i < n; i ++)
            for(int j = 0; j < m; j ++)
                if(grid[i][j])
                    q.push(i * m + j), dis[i][j] = 0;

        int res = 0;
        while(!q.empty()){
            int curx = q.front() / m, cury = q.front() % m;
            q.pop();
            res = max(res, dis[curx][cury]);

            for(int d = 0; d < 4; d ++){
                int nextx = curx + dirs[d][0], nexty = cury + dirs[d][1];
                if(in_area(nextx, nexty) && dis[nextx][nexty] == -1){
                    q.push(nextx * m + nexty);
                    dis[nextx][nexty] = dis[curx][cury] + 1;
                }
            }
        }
        return res ? res : -1;
    }

private:
    bool in_area(int x, int y){
        return x >= 0 && x < n && y >= 0 && y < m;
    }
};


int main() {

    vector<vector<int>> g1 = {{1, 0, 1}, {0, 0, 0}, {1, 0, 1}};
    cout << Solution().maxDistance(g1) << endl;
    // 2

    vector<vector<int>> g2 = {{1, 0, 0}, {0, 0, 0}, {0, 0, 0}};
    cout << Solution().maxDistance(g2) << endl;
    //4

    return 0;
}