Tree mid visit

tree visit

void root_medium(TreeNode* root) {//中序，这个难
	cout<<"中序遍历:";
	stack<TreeNode* > sta;
	TreeNode * p = root;
	while(sta.size() || p) {
		while(p) {
			sta.push(p);
			p = p->left;
		}
		if (sta.size()) {
		//	TreeNode * top = sta.top();
			p = sta.top();
			sta.pop();
			cout<<p->val<<" ";
			p = p->right;//hard here
		}
	}
	cout<<endl;
}