pat乙级1050螺旋矩阵

1、用vector建立二维数组：

 vector<vector<int>> arr(rows);
 for (int i = ; i < rows; i++)
     arr[i].resize(cols);

或：

 vector<vector<int>> v(m, vector<int>(n));

2、当数组已初始化但某些下标还未赋值时，此下标对应的变量为NULL（见34行）

代码：

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <math.h>
 using namespace std;

 bool cmp(int a, int b)
 {
     return a > b;
 }
 int main()
 {
     int n;
     cin >> n;
     vector<int> v(n);
     for (int i = ; i < n; i++)
         cin >> v[i];
     sort(v.begin(), v.end(), cmp);
     int s = sqrt(n);
     while (n % s) s--;
     int cols = s, rows = n / s;
     /*
     vector<vector<int>> arr(rows);
     for (int i = 0; i < rows; i++)
         arr[i].resize(cols);
     */
     vector<vector<int>> arr(rows, vector<int>(cols));
     int i = , j = , t = ;
     int direction = ;
     while (t < n)
     {
         if (direction == )
         {
             if (j < cols && arr[i][j] == NULL)
                 arr[i][j++] = v[t++];
             else
             {
                 i++;
                 j--;
                 direction = ;
             }
         }
         else if (direction == )
         {
             if (i < rows && arr[i][j] == NULL)
                 arr[i++][j] = v[t++];
             else
             {
                 j--;
                 i--;
                 direction = ;
             }
         }
         else if (direction == )
         {
             if (j >=  && arr[i][j] == NULL)
                 arr[i][j--] = v[t++];
             else
             {
                 i--;
                 j++;
                 direction = ;
             }
         }
         else if (direction == )
         {
             if (i >=  && arr[i][j] == NULL)
                 arr[i--][j] = v[t++];
             else
             {
                 i++;
                 j++;
                 direction = ;
             }
         }
     }
     for (int i = ; i < rows; i++)
     {
         for (int j = ; j < cols; j++)
         {
             if (j) cout << " ";
             cout << arr[i][j];
         }
         cout << endl;
     }
     return ;
 }