void PreOrder(TreeNode *root)
{
TreeNode *p = root;
stack<TreeNode*> s;
while (!s.empty() || p)
{
while (p)
{
cout << p->val << endl;
s.push(p);
p = p->left;
}
if (!s.empty())
{
p = s.top();
s.pop();
p = p->right;
}
}
}
原文：http://www.cnblogs.com/zhwcs/p/7543666.html