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 = 0; 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 = 0; 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 = 0, j = 0, t = 0;
    int direction = 1;
    while (t < n)
    {
        if (direction == 1)
        {
            if (j < cols && arr[i][j] == NULL)
                arr[i][j++] = v[t++];
            else
            {
                i++;
                j--;
                direction = 2;
            }    
        }
        else if (direction == 2)
        {
            if (i < rows && arr[i][j] == NULL)
                arr[i++][j] = v[t++];
            else
            {
                j--;
                i--;
                direction = 3;
            }
        }
        else if (direction == 3)
        {
            if (j >= 0 && arr[i][j] == NULL)
                arr[i][j--] = v[t++];
            else
            {
                i--;
                j++;
                direction = 4;
            }
        }
        else if (direction == 4)
        {
            if (i >= 0 && arr[i][j] == NULL)
                arr[i--][j] = v[t++];
            else
            {
                i++;
                j++;
                direction = 1;
            }
        }
    }
    for (int i = 0; i < rows; i++)
    {
        for (int j = 0; j < cols; j++)
        {
            if (j) cout << " ";
            cout << arr[i][j];
        }
        cout << endl;
    }
    return 0;
}