循环不变量原则——打印双Z

c++实现：“z”字用三个循环存入矩阵，每个循环都把当前的最后行/斜线的最后一个符号“*”带上，或者不带。

#include<stdio.h>
#include<iostream>
#include<sstream>
#include<vector>
#include<string>
using namespace std;


int main() {
    
    int n;
    cin >> n;
    vector<vector<char>> matrix(n+1, vector<char>(2*n+3, ' '));
    int i = 0,j=0;
    for (j = 4; j < 2 * n + 3; j += 2)
    {
        matrix[i][j] = '*';
    }
    j -= 2;
    for (; i < n; i++, j-=2)
    {
        matrix[i][j] = '*';
    }
    i--;
    j += 2;
    for (; j < 2*n+3; j += 2)
    {
        matrix[i][j] = '*';
    }
    i = 1, j = 0;
    for (; j < 2 * n -1; j += 2)
    {
        matrix[i][j] = '*';
    }
    j -= 2;
    for (; i <= n; i++, j -= 2)
    {
        matrix[i][j] = '*';
    }
    i--;
    j += 2;
    for (; j < 2 * n -1; j += 2)
    {
        matrix[i][j] = '*';
    }
    for (int i = 0; i < n - 1; i++)
    {
        for (int j = 0; j < 2*n+3-2*i; j++)
        {
            cout << matrix[i][j];
        }
        cout << endl;
    }
    for (int i = 0; i < 2 * n + 3; i++)
    {
        cout << matrix[n - 1][i];
    }
    cout << endl;
    for(int i = 0; i < 2 * n -1; i++)
    {
        cout << matrix[n][i];
    }
    cout << endl;
    return 0;
}