Whole code

#include <stdio.h>

int check(int array[8][8])
{
    int i, j, n, sum;
    int rowsum[8] = {0, 0, 0, 0, 0, 0, 0, 0};
    int colsum[8] = {0, 0, 0, 0, 0, 0, 0, 0};
    int diagsum_l2r[15] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    int diagsum_r2l[15] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

    for (i = 0; i <= 7; i++) //test if are 2 queens on the same row (i: row, j: column)
    {
        for (j = 0; j <= 7; j++)
        {
            rowsum[i] += array[i][j]; /*since they are represented by 0s and 1s,
                                    if a row's sum is bigger than 1
                                    there is more than 1 queen on that particular row
                                    here the row doesn't change until all
                                    columns are accessed (we get a row sum)*/
        }
    }


    for (i = 0; i <= 7; i++) //same as before, but for columns
    {
        for (j = 0; j <= 7; j++)
        {
            colsum[i] += array[j][i]; //here the col. doesn't change until all rows are accessed (we get a col. sum)
        }
    }

//IMPORTANT!! Diagonal sum code starts here <this is the code for the "\" (left-to-right) diagonals>
    for (n = 0; n <= 7; n++) //n counts the squares of each diagonal
    {
        sum = 0; //initialize sum to 0, here goes the sum of each diagonal line
        for (i = 0; i <= n; i++)
        {
            for (j = 0; j <= n; j++)                                    //What we basically do here is this:
            {                                               //We start from the top left corner. Then,
                if (i + j == n) //if element belongs to the main diagonal of the "sub-board"        //following the major "\" diagonal line of the main board,
                {//Observe: For all elements: i + j = n                         //we divide the board into "sub-boards" of size n.
                    sum += array[i][j]; //then add it to the general sum                //The desired sum is each time the sum of the sub-board's "/" major diagonal.
                }
            }
        }
        diagsum_l2r[n] = sum; //the sum of each diagonal goes to the array
    }

    for (n = 0; n <= 6; n++)  //same for the rest 7 diagonals
    {
        sum = 0;
        for (i = 7; i >= 0; i--)
        {
            for (j = 7; j >= 0; j--)
            {
                if (i + j == 14 - n)
                {
                    sum += array[i][j];
                }
            }
        }
        diagsum_l2r[14 - n] = sum;
    }

    for (n = 0; n <= 7; n++) //first 8 "/" diagonals (starting from the top left corner)
    {
        sum = 0;
        for (i = 7; i >= 7 - n; i--)
        {
            for (j = 0; j <= n; j++)
            {
                if (i - j == 7 - n)
                {
                    sum += array[i][j];
                }
            }
        }
        diagsum_r2l[n] = sum;
    }

    for (n = 0; n <= 6; n++) //last 7 "/" diagonals
    {
        sum = 0;
        for (i = 0; i <= 7; i++)
        {
            for (j = 7; j >= 0; j--)
            {
                if (j - i == 7 - n)
                {
                    sum += array[i][j];
                }
            }
        }
        diagsum_r2l[14 - n] = sum;
    }

    for (i = 0; i <= 7; i++)
    {
        if (rowsum[i] > 1 || colsum[i] > 1)
        {
            return 1;
        }
    }

    for (j = 0; j <= 14; j++)
    {
        if (diagsum_l2r[j] > 1 || diagsum_r2l[j] > 1)
        {
            return 1;
        }
    }

    return 0;
}


int main(void)
{
    printf("\n8 Queens Problem - Solution Validity Checker by lightspot21\n\n");
    int i = 1; //counter for the input
    int j, k, result; //counters for array printout
    int row = 0;
    int column = 0; //row and column numbers

    int board[8][8] = {
        {0, 0, 0, 0, 0, 0, 0, 0},   //here we initialize an empty board as an 8x8 array
        {0, 0, 0, 0, 0, 0, 0, 0},   //from now on: a 0 is an empty square, a 1 is a queen
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0}
    };

    while (i <= 8) //we fill our board with queens
    {
        printf("Queen #%d row:", i);
        scanf("%d", &row);
        printf("Queen #%d column:", i);
        scanf("%d", &column);
        board[row - 1][column - 1] = 1;
        i++;
    }

    printf("\nYour input is as follows: (0 = empty, 1 = queen)\n\n");
    for (j = 0; j <= 7; j++) //prints the board as in a real chessboard (first row at bottom)
    {
        printf("%d - ", j + 1);
        for (k = 0; k <= 7; k++)
        {
            printf("%d ", board[7 - j][k]);
        }
        printf("\n");
    }
    printf("    | | | | | | | |\n");
    printf("    a b c d e f g h\n");

    result = check(board);
    if (result == 1)
    {
        printf("\nThe input is not a valid solution to the 8 Queens problem.\n");
    }
    else
    {
        printf("\nThe input is a valid solution to the 8 Queens problem.\n");
    }
}