Help, adding value to array

Hi, im triying to create a editor that whenever i press the button it takes the position from the target and add it to a array list, but i cant make that the array list do a plus , there is my script:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(CameraFollow))]
public class MaxPosEditor : Editor {

    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI();

        CameraFollow camera = (CameraFollow)target;




        if (GUILayout.Button("SelectPosition"))
        {
            camera.maxPosLvl.SetValue();
            int currentpos = camera.maxPosLvl.Length - 1;
            camera.maxPosLvl[currentpos] = camera.transform.position.x;
        }
    }

}

i solved it :3, there is the code if someone want it :
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(CameraFollow))]
public class MaxPosEditor : Editor {

    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI();

        CameraFollow camera = (CameraFollow)target;




        if (GUILayout.Button("SelectPosition"))
        {
            System.Array.Resize(ref camera.maxPosLvl , camera.maxPosLvl.Length + 1);
            int currentpos = camera.maxPosLvl.Length - 1;
            camera.maxPosLvl[currentpos] = camera.transform.position.x;
        }
    }

}