已在代码中注释问题，求一个大佬来一个详细的解释。求求了~

#include <iostream>
#include <cstdio>
#pragma warning(disable:4996) 
using namespace std;

int n,m;
int dir[8][2] = { {1,0}, //↓
                  {1,1}, //↘
                  {0,1}, //→
                  {-1,1},//↗
                  {-1,0},
                  {-1,-1},
                  {0,-1},
                  {1,-1}
};

char mm[105][105];      // 存地图
bool vis[105][105];     // 标记是否访问过
                        
string goal = "yizhong";

struct node {
    int x, y;
}c[105];

void dfs(int x, int y,node c[],int k,int u)
{
    if (u == 7) {
        cout << "x: " << x << " y:" << y << " u:" << u << endl;
        // 对目标字串进行标记
        for (int i = 0; i < 7; i++) {
            vis[c[i].x][c[i].y] = 1;
        }
    }
    else {
        cout << "x: " << x << " y:" << y << " u:" << u << endl;
        int tx = x + dir[k][0];
        int ty = y + dir[k][1];
        // 问题1：为什么这里数组越界没问题
        // 问题2：为什么这里一定要加上u==6
        if ( u == 6 || mm[tx][ty] == goal[u + 1]) {
            c[u].x = x;
            c[u].y = y;
            dfs(tx,ty,c,k,u + 1);
        }
    }
}



int main() {

    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> mm[i];
    }

    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            if (mm[i][j] == 'y') {
                for (int k = 0; k < 8; k++) {
                    int tx = i + dir[k][0];
                    int ty = j + dir[k][1];
                    if (mm[tx][ty] == 'i') {

                        dfs(i,j,c,k,0);
                    }
                }
            }
        }
    }
    for (int i = 0; i < n; i++) {//输出结果
        for (int j = 0; j < n; j++)
            if (vis[i][j]) printf("%c", mm[i][j]);
            else printf("*");
        printf("\n");
    }
    
    return 0;//结束程序
}