void postOrder(TreeNode *root)
{
stack*> st;
TreeNode *p = root;
TreeNode *pre = NULL;//pre表示最近一次访问的结点
while(p || st.size()!=0)
{
//沿着左孩子方向走到最左下 。
while(p)
{
st.push(p);
p = p->left;
}
//get the top element of the stack
p = st.top();
//如果p没有右孩子或者其右孩子刚刚被访问过
if(p->right == NULL || p->right == pre)
{
//visit this element and then pop it
cout << "visit: " << p->data << endl;
st.pop();
pre = p;
p = NULL;
}
else
{
p = p->right;
}
}//end of while(p || st.size()!=0)
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