leetcode 1162. As Far from Land as Possible

class Solution {
public:
    int maxDistance(vector>& grid) {
        int n = grid.size();
        queue> q;
        for(int i=0;i
            for(int j=0;j
                if(grid[i][j] == 1){
                    q.push(make_pair(i,j));
                    grid[i][j] = 0;
                }else{
                    grid[i][j] = -1;
                }
            }
        }
        vector> dirs{{0,1},{0,-1},{1,0},{-1,0}};
        int ans = 0;
        while(!q.empty()){
            auto curr = q.front();
            q.pop();
            for(auto& d : dirs){
                int x = curr.first + d.first;
                int y = curr.second + d.second;
                if(x < 0 || y < 0 || x >= n || y >= n || grid[x][y] != -1) continue;
                q.push(make_pair(x,y));
                grid[x][y] = grid[curr.first][curr.second]+1;
                ans = max(ans,grid[x][y]);
            }
            
        }
        return ans == 0 ? -1 : ans;
    }
};

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