45. 二叉樹的後序遍歷

後續遍歷順序：

• 後序遍歷其左子樹;
• 後序遍歷其右子樹;
• 訪問根結點。

解法1：

 1 /**
 2 * Definition for a binary tree node.
 3 * struct TreeNode {
 4 *     int val;
 5 *     TreeNode *left;
 6 *     TreeNode *right;
 7 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8 * };
 9 */
10 class Solution
11 {
12 public
 
:
13     vector<int> postorderTraversal(TreeNode* root)
14     {
15         vector<int> nums;
16 
17         if (!root)
18             return nums;
19 
20         stack<TreeNode*> s;
21         TreeNode *p;
22         s.push(root);
23         p = root->left;
24 
25         while (!s.empty())
 
26         {
27             while (p != nullptr)
28             {
29                 s.push(p);
30                 p = p->left;
31             }
32 
33             bool flag = true;
34             p = nullptr;
35 
36             while (!s.empty() && flag)
37             {
38                 root = s.top();
 
39                 if (root->right == p)
40                 {
41                     s.pop();
42                     nums.push_back(root->val);
43                     p = root;
44                 }
45                 else
46                 {
47                     flag = false;
48                     p = root->right;
49                 }
50             }
51         }
52         return nums;
53     }
54 };