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：

 1 struct TreeNode {
 2     int            val;
 3     TreeNode*    left;
 4     TreeNode*    right;
 5     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 6 };
 7 
 8 vector<int> preorderTraversal(TreeNode *root) //非递归的后序遍历(用栈实现)
 9 {
10     if (NULL == root) {
11         return vector<int>();
12     }
13     
14     stack<TreeNode *> tree_stack;
15     vector<int> tree_vector;
16 
17     tree_stack.push(root);
18     TreeNode *p = NULL;
19     while (!tree_stack.empty()) {
20         TreeNode *pTemp = tree_stack.top();
21         if ((pTemp->left == NULL && pTemp->right == NULL) || ((p != NULL) && (pTemp->left == p || pTemp->right == p))) {
22             tree_vector.push_back(pTemp->val);
23             tree_stack.pop();
24             
25             p = pTemp;
26         }
27         else {
28             while(pTemp) {
29                 if (pTemp->right != NULL) 
30                     tree_stack.push(pTemp->right);
31 
32                 if (pTemp->left != NULL)
33                     tree_stack.push(pTemp->left);
34                 pTemp = pTemp->left;
35             }
36         }
37     }
38     return tree_vector;
39 }