三维BFS，直接搬模板就好

#include <bits/stdc++.h>

using namespace std;

const int N = 110;

int n;
char m[N][N][N];
int dis[N][N][N];

typedef struct Edge
{
    int third;
    int second;
    int first;
}three;

int bfs()
{
    queue<three> q;
    memset(dis, -1, sizeof dis);
    q.push({1, 1, 1});
    dis[1][1][1] = 1;
    int dx[] = {0, 1, 0, -1 ,0 , 0}, dy[] = {1, 0, -1, 0, 0, 0}, dz[] = {0, 0, 0, 0, 1, -1};
    while(q.size())
    {
        three t = q.front();
        q.pop();
        for(int i = 0; i < 6; i++)
        {
            int x = t.third + dx[i], y = t.second + dy[i], z = t.first + dz[i];
            if(x >= 1 && y >= 1 && z >= 1 && x <= n && y <= n && z <= n && dis[x][y][z] == -1 && m[x][y][z] != '*')
            {
                dis[x][y][z] = dis[t.third][t.second][t.first] + 1;
                q.push({x, y, z});
            }
        }
    }

    return dis[n][n][n];
}

int main()
{
    cin >> n;
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= n; j++)
            for(int k = 1; k <= n; k++)
                cin >> m[i][j][k];

    auto t = bfs();
    if(t == -1)
        cout << -1 << endl;
    else
        cout << t << endl;

    return 0;
}