1515
1616#### TreeNode 定义
1717
18+ ``` python
19+ class TreeNode :
20+ def __init__ (self val = 0 , left = None , right = None ):
21+ self .val = val
22+ self .left = left
23+ self .right = right
24+ ```
25+ 1826``` cpp
1927struct TreeNode {
2028 int val;
@@ -26,17 +34,35 @@ struct TreeNode{
2634```
2735
2836#### 前序递归
37+ ```python
38+ # Definition for a binary tree node.
39+ # class TreeNode:
40+ # def __init__(self, val=0, left=None, right=None):
41+ # self.val = val
42+ # self.left = left
43+ # self.right = right
44+ class Solution:
45+ def preOrderRecursion(self, root, res):
46+ if root is None: return
47+ res.append(root.val) #调整append位置,就是前/中/后序遍历的差别
48+ self.preOrderRecursion(root.left, res)
49+ self.preOrderRecursion(root.right, res)
50+ def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
51+ res = list()
52+ self.preOrderRecursion(root, res)
53+ return res
54+ ```
2955
3056``` cpp
3157// 前中后序遍历只在这个函数有区别, 调整push_back的顺序
3258void preOrderRecursion (TreeNode* root, vector<int >& res){
59+ if(root == nullptr) return {};
3360 res.push_back(root->val);
3461 preOrderRecursion(root->left, res);
3562 preOrderRecursion(root->right, res);
3663}
3764
3865vector<int > preOrderTraversal(TreeNode* root){
39- if(root == nullptr) return {};
4066
4167 vector<int > res;
4268
@@ -46,7 +72,11 @@ vector<int> preOrderTraversal(TreeNode* root){
4672}
4773```
4874
75+
4976#### 前序非递归
77+ ```python
78+
79+ ```
5080
5181``` cpp
5282// 通过非递归遍历: 用栈进行辅助
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