前序遍历和中序遍历的模版，记住它你的人生是彩色的

中序遍历

TreeNode* KthNode(TreeNode* head, int k){
        std::stack<TreeNode*> mstck;
        TreeNode* curr = head;
        while(curr || !mstck.empty()){
            while(curr){
                mstck.push(curr);
                curr = curr->left;
            }

            if(--k == 0)
                return mstck.top();
            //访问节点
            curr = mstck.top()->right;
            mstck.pop();
        }
        return nullptr;
    }

前序遍历

TreeNode* KthNode(TreeNode* head, int k){
        std::stack<TreeNode*> mstck;
        TreeNode* curr = head;
        while(curr || !mstck.empty()){
            while(curr){
                //访问节点
                mstck.push(curr);
                curr = curr->left;
            }

            .........................

            curr = mstck.top()->right;
            mstck.pop();
        }
        return nullptr;
    }