Common ancestor of binary tree

// Combine the two public sectors holding ancestors into one
// Merged ancestors, the order is reversed, that is, the last one is the most recent ancestor
void mergeCommon(char stack[],  char stack1[], char stack2[], int top1, int top2) {
    for (int i = 0; i <= top1 && i <= top2; i++) {
        if (stack1[i] == stack2[i]) {
            stack[i] = stack1[i];
        } else {
            break;
        }
    }
}

void copyStack(BTNode *stack1[], char stack2[], int top) {
    for (int i = 0; i <= top; i++) {
        stack2[i] = stack1[i]->data;
     }
}

// Find the common ancestor idea of ​​two nodes, use post-order non-recursive algorithm
// Use two auxiliary stacks, one stack to save
void printCommonAncestor(BTNode *root, char y, char z) {
    if (root != nullptr) {
        int tag[20]; // mark whether the right subtree of each node has been visited
        BTNode *stack[maxsize];
        char stacky[20], stackz[20]; // Auxiliary stack, keep the ancestor of node y and the ancestor of node z
        int topy = -1, topz = -1;
        int top = -1;
        BTNode *q;
        q = root;
        while (q != nullptr || top != -1) {
            while (q != nullptr) {
                stack[++top] = q;
                q = q->left;
                tag[top] = 0;
            }
            while (top != -1 && tag[top] == 1) {
                q = stack[top--];
                if (q->data == y) {
                    // Copy the rest of the stack to stacky
                    copyStack(stack, stacky, top);
                    topy = top;
                }
                if (q->data == z) {
                    copyStack(stack, stackz, top);
                    topz = top;
                }
            }
            if (top != -1) {
                tag[top] = 1;
                q = stack[top];
                q = q->right;
            } else {
                break;
            }
        }

        // Finally, stacky and stackz save their ancestors
        char commonAncestor[maxsize];
        mergeCommon (commonAncestor, stacks, stackz, tops, topz);
        printf("");
    }
}