求助，最后一个点RE

#include <iostream>
#include <bits/stdc++.h>

using namespace std;
int main(int argc, char** argv) {
    string ans[10];
    for (int i = 0;i < 10;i ++) {
        ans[i] = "";
    }
    int n;
    cin>>n;
    char a[1000000];
    for (int i = 1;i <= n;i ++) {
        cin>>a[i];
        switch (a[i]) {
            case '0':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "X.X.";
                ans[3] = ans[3] + "X.X.";
                ans[4] = ans[4] + "X.X.";
                ans[5] = ans[5] + "XXX.";
                break;
            case '1':
                ans[1] = ans[1] + "..X.";
                ans[2] = ans[2] + "..X.";
                ans[3] = ans[3] + "..X.";
                ans[4] = ans[4] + "..X.";
                ans[5] = ans[5] + "..X.";
                break;
            case '2':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "..X.";
                ans[3] = ans[3] + "XXX.";
                ans[4] = ans[4] + "X...";
                ans[5] = ans[5] + "XXX.";   
                break;
            case '3':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "..X.";
                ans[3] = ans[3] + "XXX.";
                ans[4] = ans[4] + "..X.";
                ans[5] = ans[5] + "XXX.";
                break;
            case '4':
                ans[1] = ans[1] + "X.X.";
                ans[2] = ans[2] + "X.X.";
                ans[3] = ans[3] + "XXX.";
                ans[4] = ans[4] + "..X.";
                ans[5] = ans[5] + "..X.";
                break;
            case '5':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "X...";
                ans[3] = ans[3] + "XXX.";
                ans[4] = ans[4] + "..X.";
                ans[5] = ans[5] + "XXX.";
                break;
            case '6':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "X...";
                ans[3] = ans[3] + "XXX.";
                ans[4] = ans[4] + "X.X.";
                ans[5] = ans[5] + "XXX.";
                break;
            case '7':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "..X.";
                ans[3] = ans[3] + "..X.";
                ans[4] = ans[4] + "..X.";
                ans[5] = ans[5] + "..X.";
                break;
            case '8':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "X.X.";
                ans[3] = ans[3] + "XXX.";
                ans[4] = ans[4] + "X.X.";
                ans[5] = ans[5] + "XXX.";
                break;
            case '9':
                ans[1] = ans[1] + "XXX.";
                ans[2] = ans[2] + "X.X.";
                ans[3] = ans[3] + "XXX.";
                ans[4] = ans[4] + "..X.";
                ans[5] = ans[5] + "XXX.";
                break;
            default:
                break;
        }
    }
    //output.
    for (int i = 1;i <= n;i ++) {
        ans[i] = ans[i].substr(0,ans[i].length() - 1);
        cout<<ans[i]<<endl;
    }
    return 0;
}

在本地运行的时候，当第一个数字超过10时，提示Segmentation Fault，不知道是哪里出问题了。