二叉树的统一迭代遍历法

vector<int> orderTraversal(TreeNode* root) {
        stack<TreeNode*> s;
        vector<int> result;
        if(!root) return result;
        else s.push(root);
        while(!s.empty()){
            TreeNode *curPtr = s.top();
            if(curPtr){
                s.pop();
/*  只需要改变他们的顺序即可决定是前序、后序还是中序
                if(curPtr->right) s.push(curPtr->right);  //右
                s.push(curPtr);  //中
                s.push(nullptr); //中
                if(curPtr->left) s.push(curPtr->left);    //左
*/
            }else{
                s.pop();
                curPtr = s.top();
                result.push_back(curPtr->val);
                s.pop();
            }
        }
        return result;
    }