二叉树 后续 非递归

#include <stack>
#include <iostream>
using namespace std;
template <class T>
class TreeNode
{
  public:
    T data;
    TreeNode<T> *left; //left child
    TreeNode<T> *right; //right child
 
    TreeNode():left(NULL),right(NULL)
    {
    }
    TreeNode(const T& t):data(t),left(NULL), right(NULL)
    {
    }
    TreeNode(const T& t, TreeNode<T*> left, TreeNode<T*> right):data(t),left(left), right(left)
    {
    }
};

template <class T>
void postOrder(TreeNode<T> *root)
{
  stack<TreeNode<T>*> st;
  TreeNode<T> *p = root;
  TreeNode<T> *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)
}