Lesson 1: serpentine matrix

subject ：AcWing 756. Snake matrix
Enter two integers n and m, Output one n That's ok m Columns of the matrix , The digital 1 To n×m Fill the matrix in the shape of a serpentine .

For the specific matrix form, please refer to the example .

Input format
Enter a total of one line , Contains two integers n and m.

Output format
Output a matrix that meets the requirements .

Matrix occupation n That's ok , Each row contains m An integer separated by spaces .

Data range
1≤n,m≤100

sample input ：

3 3

sample output ：

1 2 3
8 9 4
7 6 5

Topic analysis ：
This question mainly uses x、y The offset
such as （x,y） If you want to go down, it's （x+1,y+0）, The same goes for other directions
If you change direction ？
Use an array to save the offsets in four directions , If you change the direction of the array subscript +1 model 4 that will do .

#include <iostream>
using namespace std;

const int N = 105;
int a[N][N];
int dx[]={
    0,1,0,-1}; //  Right down left up 
int dy[]={
    1,0,-1,0};
int main()
{
    
    int n,m;
    cin>>n>>m;
    int x=0,y=0,d=0;
    for(int i=1;i<=n*m;i++)
    {
    
        a[x][y]=i;
        x+=dx[d],y+=dy[d]; //  For mobile 
        if(x<0||y<0||x>=n||y>=m||a[x][y])
        {
    
            x-=dx[d],y-=dy[d]; // If the Movement fails, it needs to be restored 
            d=(d+1)%4;
            x+=dx[d],y+=dy[d]; //  Move... Again 
        }
    }
    for(int i=0;i<n;i++)
    {
    
        for(int j=0;j<m;j++)cout<<a[i][j]<<' ';
        cout<<endl;
    }
        
    return 0;
}