请输入层数: 10
0
12
345
6789
01234
567890
1234567
89012345
678901234
5678901234
int line_number, now_number;
int i,row;

cout << "请输入行数: ";
cin >> line_number;
now_number = 0;

for (row = 1; row < line_number + 1; row++){ // L1
    for (i = 0; i < row; i++){ // L2
        cout << now_number;
        now_number += 1;
        if (now_number == 10) now_number = 0; // L3
    }
    cout << endl;
}