Duplicate subtree

You are given a binary tree. You have to find all the duplicate sub-trees of the binary tree. For every duplicate subtree, you just have to return the root of the sub-tree. Two sub-trees are duplicates or the same if the nodes have the same value and the same structure.

Example:-

For the given binary tree: [1, 2, 1, -1, -1, 2, 1, -1, -1, -1, -1]
Start Node: 3

    1
   / \
 2   1
      / \
     2  1

The answer should be [[2]] because the subtree [2] is present 2 times.

Input Format :

The first line contains a single integer ‘T’ representing the number of test cases. Then each test case follows.

The first line of each test case contains elements of the tree in the level order form. The line consists of values of nodes separated by a single space. In case a node is null, we take -1 in its place.

For example, the input for the tree depicted in the above image would be :

1
2 3
4 -1 5 6
-1 7 -1 -1 -1 -1
-1 -1

Explanation :

Level 1 :
The root node of the tree is 1

Level 2 :
Left child of 1 = 2
Right child of 1 = 3

Level 3 :
Left child of 2 = 4
Right child of 2 = null (-1)
Left child of 3 = 5
Right child of 3 = 6

Level 4 :
Left child of 4 = null (-1)
Right child of 4 = 7
Left child of 5 = null (-1)
Right child of 5 = null (-1)
Left child of 6 = null (-1)
Right child of 6 = null (-1)

Level 5 :
Left child of 7 = null (-1)
Right child of 7 = null (-1)

Note:

The above format was just to provide clarity on how the input is formed for a given tree. 
The sequence will be put together in a single line separated by a single space. 
Hence, for the above-depicted tree, the input will be given as:
1 2 3 4 -1 5 6 -1 7 -1 -1 -1 -1 -1 -1

Output Format :

For each test case, print the subtrees that are present more than once in the tree.

The output of each test case should be printed in a separate line.

Note :

You are not required to print anything, it has already been taken care of. Just implement the function.    

Constraints :

1 <= T <= 5
1 <= N (Number of Nodes) <= 10^5
1 <= VALUE of the nodes <= 10^9

Time Limit = 1 sec