Binary Tree 2 Linked List

I am not asking any solution but please tell me why g++ compiler gives
me the following error. I am using g++ as against gcc only because I
used #include<stack> from C++ STL to store the values in stack as
opposed to creating one of mine. All of my code is in C.

I am converting a Binary Tree to a Doubly Linked List by using an
inorder traversal and changing pointers.

btree struct looks like this

typedef struct btree{
        struct btree* left;
        int data;
        struct btree* right;

} btree;

And this is my function, where I get the error from g++ compiler

btree* ConvertBinaryTree2LinkedList(btree* root){
        btree *cursor = root;
        btree *head = NULL; // return as first element of linked list
        int isLeftMost = 1;
        stack < btree* > S;
        int done = 0;

        // Iterative Inorder Traversal
        while(!done){
                if (cursor != NULL) {
                        S.push(cursor);
                        cursor=cursor->left;
                        if (isLeftMost) head = cursor;
                }
                else{
                        if(!S.empty()){
                                isLeftMost = 0;
                                cursor = S.top();
                                S.pop();
                                cursor->right = S.top();
                                S.top()->left = cursor;
                                cursor = cursor->right;
                        }
                        else{
                                done = 1;
                        }
                }
        }
        return head;

}

BinaryTree2LinkedList.cpp: In function 'btree*
ConvertBinaryTree2LinkedList(btree*)':
BinaryTree2LinkedList.cpp:72: error: 'stack' was not declared in this
scope
BinaryTree2LinkedList.cpp:72: error: expected primary-expression
before '*' token
BinaryTree2LinkedList.cpp:72: error: expected primary-expression
before '>' token
BinaryTree2LinkedList.cpp:72: error: 'S' was not declared in this
scope

Please help to fix the error. Every help is appreciated.