二叉樹公共祖先

// 將兩個保存祖先的公共部門組合併成一個
// 合併後的祖先，順序是反的，就是最後一個纔是最近的祖先
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;
     }
}

// 尋找兩個節點的共同祖先 思路，用後序非遞歸算法
// 用兩個輔助棧，一個棧保存
void printCommonAncestor(BTNode *root, char y, char z) {
    if (root != nullptr) {
        int tag[20]; // 標記每一個節點的右子樹是否被訪問過
        BTNode *stack[maxsize];
        char stacky[20], stackz[20]; // 輔助棧，保留節點y的祖先和節點z的祖先
        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) {
                    // 將棧裏面剩餘的複製到stacky裏面
                    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;
            }
        }

        // 最後stacky 和 stackz裏面保存的都是各自的祖先
        char commonAncestor[maxsize];
        mergeCommon(commonAncestor, stacky, stackz, topy, topz);
        printf("");
    }
}