94.二叉树中序遍历

    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode *> st;
        while(1) {
            if (root) {
                st.push(root);
                root = root->left;
            } else if (!st.empty()) {
                TreeNode *p = st.top();
                st.pop();
                res.push_back(p->val);
                root=p->right;
            } else break;
        }
        return res;
    }