LeetCode：145_Binary Tree Postorder Traversal | 二叉树后序遍历 | Hard

题目：Binary Tree Postorder Traversal

二叉树的后序遍历，题目要求是采用非递归的方式，这个在上数据结构的课时已经很清楚了，二叉树的非递归遍历不管采用何种方式，都需要用到栈结构作为中转，代码很简单，见下：

 struct TreeNode {
     int            val;
     TreeNode*    left;
     TreeNode*    right;
     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 };

 vector<int> preorderTraversal(TreeNode *root) //非递归的后序遍历(用栈实现)
 {
     if (NULL == root) {
         return vector<int>();
     }

     stack<TreeNode *> tree_stack;
     vector<int> tree_vector;

     tree_stack.push(root);
     TreeNode *p = NULL;
     while (!tree_stack.empty()) {
         TreeNode *pTemp = tree_stack.top();
         if ((pTemp->left == NULL && pTemp->right == NULL) || ((p != NULL) && (pTemp->left == p || pTemp->right == p))) {
             tree_vector.push_back(pTemp->val);
             tree_stack.pop();

             p = pTemp;
         }
         else {
             while(pTemp) {
                 if (pTemp->right != NULL)
                     tree_stack.push(pTemp->right);

                 if (pTemp->left != NULL)
                     tree_stack.push(pTemp->left);
                 pTemp = pTemp->left;
             }
         }
     }
     return tree_vector;
 }