二叉树前序与中序遍历非递归

class Solution {
public:
    vector preorderTraversal(TreeNode* root) {
        vector preorder;
        vector inorder;
        stack s;
	    TreeNode *cur = root;

	    while (cur != NULL || !s.empty()) {
		    while (cur != NULL) {
			    s.push(cur);
			    preorder.emplace_back(cur->val);//这里输出是前序遍历
			    cur = cur->left;
		    }

		    TreeNode *top = s.top();
		    s.pop();
            inorder.emplace_back(top->val);//这里输出是中序遍历
		    cur = top->right;
        }
        return v;
    }
};