View Compilation Error

0_0_25173392_13608.cpp: In function 'Node* new_tree(Node*, int)':
0_0_25173392_13608.cpp:20:7: error: 'a' was not declared in this scope
   t = a[x++];
       ^
0_0_25173392_13608.cpp: In function 'int main()':
0_0_25173392_13608.cpp:65:8: error: 'a' was not declared in this scope
    if (a[i].left != NULL || a[i].right != NULL) {
        ^