服了，最后一行str读完了，必须要用空格填充

C++ 代码

#include<iostream>
#include<string>
using namespace std;
const int N = 1010;
int n;
string str;
char c[N][N];
int main()
{
    cin >> n;
    getchar();      //读入回车
    getline(cin, str);
    int m = str.size();
    int w;
    int k = 0;
    if (m % n == 0)w = m / n;
    else w = m / n + 1;
    for (int i = w; i >= 1; i--) {  
        for (int j = 1; j <= n; j++) {  
            if (k < m) {  
                c[j][i] = str[k++];  
            } else {  
                c[j][i] = ' '; // 如果str已读完，用空格填充  
            }  
        }  
    }  
    for (int i = 1; i <= n; i++)
    {
        for (int j = 1; j <= w; j++)
        {
            cout << c[i][j];
        }
        cout << endl;
    }
    return 0;
}