一运行就出错的程序……【求指点具体错误或者算法……】

恩啊，近期在做Euler Project，其中有一题是这样的：

Largest product in a grid


Problem 11
 

In the 2020 grid below, four numbers along a diagonal line have been marked in red.
 
08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
 49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00
 81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65
 52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91
 22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80
 24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50
 32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70
 67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21
 24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
 21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95
 78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92
 16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57
 86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58
 19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40
 04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66
 88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69
 04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36
 20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16
 20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54
 01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48
 

The product of these numbers is 26 63 78 14 = 1788696.
 
What is the greatest product of four adjacent numbers in the same direction (up, down, left, right, or diagonally) in the 2020 grid?


于是我就用循环写了这么一个程序：
#include <iostream>

using namespace std;

int main()
{
    const int a[20][20]=
{
 { 8,  2, 22, 97, 38, 15,  0, 40,  0, 75,  4,  5,  7, 78, 52, 12, 50, 77, 91,  8},
 {49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48,  4, 56, 62,  0},
 {81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30,  3, 49, 13, 36, 65},
 {52, 70, 95, 23,  4, 60, 11, 42, 69, 24, 68, 56,  1, 32, 56, 71, 37,  2, 36, 91},
 {22, 31, 16, 71, 51, 67, 63, 89, 41, 92, 36, 54, 22, 40, 40, 28, 66, 33, 13, 80},
 {24, 47, 32, 60, 99,  3, 45,  2, 44, 75, 33, 53, 78, 36, 84, 20, 35, 17, 12, 50},
 {32, 98, 81, 28, 64, 23, 67, 10, 26, 38, 40, 67, 59, 54, 70, 66, 18, 38, 64, 70},
 {67, 26, 20, 68,  2, 62, 12, 20, 95, 63, 94, 39, 63,  8, 40, 91, 66, 49, 94, 21},
 {24, 55, 58,  5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72},
 {21, 36, 23,  9, 75,  0, 76, 44, 20, 45, 35, 14,  0, 61, 33, 97, 34, 31, 33, 95},
 {78, 17, 53, 28, 22, 75, 31, 67, 15, 94,  3, 80,  4, 62, 16, 14,  9, 53, 56, 92},
 {16, 39,  5, 42, 96, 35, 31, 47, 55, 58, 88, 24,  0, 17, 54, 24, 36, 29, 85, 57},
 {86, 56,  0, 48, 35, 71, 89,  7,  5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58},
 {19, 80, 81, 68,  5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77,  4, 89, 55, 40},
 { 4, 52,  8, 83, 97, 35, 99, 16, 07, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66},
 {88, 36, 68, 87, 57, 62, 20, 72,  3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69},
 { 4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18,  8, 46, 29, 32, 40, 62, 76, 36},
 {20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74,  4, 36, 16},
 {20, 73, 35, 29, 78, 31, 90,  1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57,  5, 54},
 { 1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52,  1, 89, 19, 67, 48}};


    const int N = 20;

    int i, j;
    long int curr,
             maxi[5] = { 0, 0, 0, 0, 0};

    curr = 0;

    for(i = 0;i < N;i ++)
    {
        for(j = 0;j < N-4;j ++)
        {
            curr = a[i][j+0]
                  *a[i][j+1]
                  *a[i][j+2]
                  *a[i][j+3];
            maxi[1] = curr>maxi[1]?curr:maxi[1];
        }
    }
    cout << "Hello world!" << endl << "maxi[1] " << maxi[1] << endl;

    for(j = 0;j < N;i ++)
    {
        for(i = 0;i < N-4;j ++)
        {
            curr = a[i+0][j]
                  *a[i+1][j]
                  *a[i+2][j]
                  *a[i+3][j];
            maxi[2] = curr>maxi[2]?curr:maxi[2];
        }
    }
    cout << "Hello world!" << endl << "maxi[2] " << maxi[2] << endl;

    for(i = 0;i < N-4;i ++)
    {
        for(j = 0;j < N-4;j ++)
        {
            curr = a[i+0][j+0]
                  *a[i+1][j+1]
                  *a[i+2][j+2]
                  *a[i+3][j+3];
            maxi[3] = curr>maxi[3]?curr:maxi[3];
        }
    }
    cout << "Hello world!" << endl << "maxi[3] " << maxi[3] << endl;

    for(i = 0;i < N-4;i ++)
    {
        for(j = 0;j < N-4;j ++)
        {
            curr = a[i+3][j+0]
                  *a[i+2][j+1]
                  *a[i+1][j+2]
                  *a[i+0][j+3];
            maxi[4] = curr>maxi[4]?curr:maxi[4];
        }
    }
    cout << "Hello world!" << endl << "maxi[4] " << maxi[4] << endl;


    return 0;
}

最初maxi不是一个数组，一运行显示第一个maxi之后就报错，所以改成数组；但是还是一输出maxi[1]就出错……
求指点……