Unity 2Dでパズルを重ならないように置きたい

Unity2Dでドラッグ＆ドロップではめていくパズルゲームを作成中です。
現在はめたときにパズルが重なってしまうので、パズルを重ならないようにしたいです。

Group.cs

C#
1using System.Collections;
2using System.Collections.Generic;
3using UnityEngine;
4
5public class Group : MonoBehaviour
6{
7    private Vector3 screenPoint;
8    private Vector3 offset;
9
10    bool isValidGridPos()
11    {
12        foreach (Transform child in transform)
13        {
14            Vector2 v = Grid.roundVec2(child.position);
15
16            if (!Grid.insideBorder(v))
17                return false;
18
19            if (Grid.grid[(int)v.x, (int)v.y] != null &&
20               Grid.grid[(int)v.x, (int)v.y].parent != transform)
21                return false;
22        }
23        return true;
24    }
25
26    void updateGrid()
27    {
28        for (int y = 0; y < Grid.h; ++y)
29            for (int x = 0; x < Grid.w; ++x)
30                if (Grid.grid[x, y] != null)
31                    if (Grid.grid[x, y].parent == transform)
32                        Grid.grid[x, y] = null;
33
34        foreach (Transform child in transform)
35        {
36            Vector2 v = Grid.roundVec2(child.position);
37            Grid.grid[(int)v.x, (int)v.y] = child;
38        }
39    }
40
41    void OnMouseDown()
42    {   
43        // マウスカーソルは、スクリーン座標なので、
44        // 対象のオブジェクトもスクリーン座標に変換してから計算する。
45
46        // このオブジェクトの位置(transform.position)をスクリーン座標に変換。
47        screenPoint = Camera.main.WorldToScreenPoint(transform.position);
48        // ワールド座標上の、マウスカーソルと、対象の位置の差分。
49        offset = transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
50
51        
52    }
53
54    void OnMouseDrag()
55    {
56        Vector3 currentScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
57        Vector3 currentPosition = Camera.main.ScreenToWorldPoint(currentScreenPoint) + this.offset;
58        transform.position = currentPosition;
59    }
60
61    void OnMouseUp()
62    {
63        Vector3 currentScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
64        Vector3 currentPosition = Camera.main.ScreenToWorldPoint(currentScreenPoint) + this.offset;
65        
66
67        if (isValidGridPos())
68        {
69            transform.position = Grid.roundVec2(currentPosition);
70
71            Grid.deleteFullRows();
72
73            FindObjectOfType<Spawner>().spawnNext();
74            
75        }
76        else
77        {
78            transform.position = GameObject.Find("Spawner").transform.position;
79        }
80    }
81
82
83    // Use this for initialization
84    void Start()
85    {
86        if (!isValidGridPos())
87        {
88            Debug.Log("GAME OVER");
89        }
90    }
91
92    // Update is called once per frame
93    void Update()
94    {
95   
96    }
97}
98
99

Grid.cs

C#
1using System.Collections;
2using System.Collections.Generic;
3using UnityEngine;
4
5public class Grid: MonoBehaviour {
6    public static int w = 10;
7    public static int h = 20;
8    public static Transform[,] grid = new Transform[w, h];
9
10    public static Vector2 roundVec2(Vector2 v)
11    {
12        return new Vector2(Mathf.Round(v.x),
13                           Mathf.Round(v.y));
14    }
15
16    public static bool insideBorder(Vector2 pos)
17    {
18        return ((int)pos.x >= 0 &&
19               (int)pos.x < w &&
20               (int)pos.y >= 0);
21    }
22
23    public static void deleteRow(int y)
24    {
25        for(int x = 0; x < w; ++x)
26        {
27            Destroy(grid[x, y].gameObject);
28            grid[x, y] = null;
29        }
30    }
31
32    public static void deleteLine(int x)
33    {
34        for (int y = 0; y < h; ++x)
35        {
36            Destroy(grid[x, y].gameObject);
37            grid[x, y] = null;
38        }
39    }
40
41    public static bool isRowFull(int y)
42    {
43        for (int x = 0; x < w; ++x)
44            if (grid[x, y] == null)
45                return false;
46        return true;
47    }
48
49    public static void deleteFullRows()
50    {
51        for(int y = 0; y < h; ++y){
52            if (isRowFull(y))
53            {
54                deleteRow(y);
55            }
56        }
57    }
58}

Spawner.cs

C#
1using System.Collections;
2using System.Collections.Generic;
3using UnityEngine;
4
5public class Spawner : MonoBehaviour
6{
7
8    public GameObject[] groups;
9    public void spawnNext()
10    {
11        int i = Random.Range(0, groups.Length);
12
13        Instantiate(groups[i],
14                    transform.position,
15                    Quaternion.identity);
16    }
17
18    // Use this for initialization
19    void Start()
20    {
21        spawnNext();
22    }
23
24    // Update is called once per frame
25    void Update()
26    {
27
28    }
29}