題意：

非遞迴中序遍歷。

樹的先序遍歷

vector<int> preorderTraversal(TreeNode* root) {
	stack<TreeNode*> s;
	vector<int> res;
	TreeNode *p = root;
	while (p || !s.empty()) {
		while (p) {
			s.push(p);
			//res.push_back(p->val);
			p = p->left;
		}
		if (!s.empty()) {
			p = s.top();
 

			s.pop();
			p = p->right;
		}
	}
	return res;
}

中序遍歷

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

後序遍歷

void PostOrder(TreeNode *root) {
    TreeNode *p = root, *r = NULL;
    stack<TreeNode*> s;
    while (p || !s.empty()) {
        if (p) {//走到最左邊
            s.push(p);
            p = p->left;
        }
 

        else {
            p = s.top();
            if (p->right && p->right != r)//右子樹存在，未被訪問
                p = p->right;
            else {
                s.pop();
                visit(p->val);
                r = p;//記錄最近訪問過的節點
                p = NULL;//節點訪問完後，重置p指標
            }
        }//else
    }//while
}