[BFS] leetcode 1091 Shortest Path in Binary Matrix

problem:https://leetcode.com/problems/shortest-path-in-binary-matrix/

        Very basic single-source shortest path has no right, no change.

int dx[] = {1,0,-1,0,1,-1,1,-1};
int dy[] = {0,1,0,-1,1,1,-1,1};
class Solution {
public:
    int shortestPathBinaryMatrix(vector<vector<int>>& grid) {
        int s = grid.size() * grid.size();
        int n = grid.size();
        if(grid[n-1][n-1]==1)return -1;
        if(grid[0][0]==1)return -1;
        if(n >= 2)
        {
            if(grid[n-2][n-2]==1 && grid[n-1][n-2]==1&&grid[n-2][n-1]==1)return -1;
        }
        vector<int> dist(s,INT_MAX);
        queue<int> q;
        q.push(0);
        dist[0] = 1;
        while(!q.empty()) {
            int v = q.front();q.pop();
        
            int i = v / n;
            int j = v % n;
            for(int k = 0;k < 8;k++) {
                int x0 = i + dx[k];
                int y0 = j + dy[k];
                int t = x0 * n + y0;
          
                if(x0 >=0 && y0 >=0 && x0 < n && y0 < n && grid[x0][y0] == 0 && dist[t] == INT_MAX)
                {
                    dist[t] = dist[v] + 1;
                    q.push(t);
               }
            }
        }
        if(dist[s-1] == INT_MAX) return -1;
        return dist[s - 1];
    }
};