二叉树后序遍历-非递归算法

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode * right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

vector<int> postorderTraversal(TreeNode* root) {
        vector<int> path;

        stack<TreeNode *> st;
        TreeNode * pre = NULL;

        bool leftTag;
        do
        {
            //root及其所有的左子树全部入栈
            while (root != NULL)
            {
                st.push(root);
                root = root->left;
            }

            leftTag = true; // leftTag = ture, 说明左子树为NULL或者已经全部被访问
            pre = NULL; //

            while (!st.empty() && leftTag)
            {
                root = st.top();
                if (root->right == pre)
                {
                    //访问栈顶元素
                    path.push_back(root->val);
                    //出栈
                    st.pop();

                    pre = root;

                }
                else
                {
                    leftTag = false;
                    root = root->right;
                }
            }

        } while (!st.empty());

        return path;
    }