Zigzag conversion on a string

zigzag_conversion.c

char *zigzag_convert(char *s, int numRows)
{
    int numChars   = strlen(s);
    if (numRows == 1) return s; // Division by zero will occur in the next line otherwise.
    int numColumns = numChars / (numRows + numRows - 2) * (numRows - 1) + numChars % (numRows + numRows - 2) / numRows + numChars % (numRows + numRows - 2) % numRows;

    // Allocate space for the table.
    char **table = malloc(numRows * sizeof(char *));
    for (int i = 0; i < numRows; i++)
    {
        table[i] = malloc(numColumns * sizeof(char));
    }

    int i;              // Index indicating the row number.
    int j;              // Index indicating the column number.
    int numCharsPlaced; // The number of characters placed either in the table or the string.
    bool writeDownward; // The direction characters are being added to the table.

    // Ensure all characters are initially NULL.
    for (i = 0; i < numRows; i++)
    {
        for (j = 0; j < numColumns; j++)
        {
            table[i][j] = NULL;
        }
    }

    i = 0;
    j = 0;
    numCharsPlaced = 0;
    writeDownward  = true;
    // Add all characters from the string to the table.
    while(numCharsPlaced != numChars)
    {
        if   (i == numRows)
        {
            writeDownward   = false;
            i -= 2;
            j++;
        }
        if   (i == 0)
        {
            writeDownward   = true;
        }
        if   (writeDownward)
        {
            table[i++][j]   = s[numCharsPlaced++];
        }
        else
        {
            table[i--][j++] = s[numCharsPlaced++];
        }
    }

    numCharsPlaced = 0;
    // Add all characters from the table to the string.
    for (i = 0; i < numRows; i++)
    {
        for (j = 0; j < numColumns; j++)
        {
            if (table[i][j])
            {
                s[numCharsPlaced++] = table[i][j];
            }
        }
    }

    return s;
}