中序遍歷二叉樹分為兩種方法，第一種是遞歸法，第二種是迭代法，遞歸法在LeetCode OJ上被稱為是trivial（不重要的），所以重點在迭代法遍歷二叉樹

• 遞歸法

struct TreeNode {
    int value;
    TreeNode *leftTreeNode;
    TreeNode *rightTreeNode;
};
void traverseInOrder (TreeNode *treeNode) {
    if (treeNode != NULL) {
        traverseInOrder(treeNode->leftTreeNode);
        std::cout << treeNode->value;
        traverseInOrder(treeNode->rightTreeNode);
    }
}

• 迭代法

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
};
vector<int> traverseInOrder (TreeNode *root) {
    vector<int> allValueInOrder;
    TreeNode *currentNode = root;
    stack<TreeNode *> stack;
    while (currentNode || !stack.empty()) {// isEmpty類型為變量為空返回YES，所以一般判斷都應該取反
        if (currentNode) {
            stack.push(currentNode);
            currentNode = currentNode->left;
        } else {
            currentNode = stack.top();
            stack.pop();// pop返回了void，不能直接進行pop返回數(shù)據(jù)
            allValueInOrder.push_back(currentNode->val);
            currentNode = currentNode->right;
        }
    }
    return allValueInOrder;
}