【树】144. 二叉树的前序遍历

题目：

解法：
方法一：递归方法

 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 public:
12     vector<int> preorderTraversal(TreeNode* root) 
13     {
14         vector<int> ret;
15         if (NULL == root)
16         {
17             return ret;
18         }
19 
20         inorderTraversalHelper(root, ret);
21         return ret;
22     }
23 
24     void inorderTraversalHelper(TreeNode * root, vector<int> &ret)
25     {
26         if (NULL != root)
27         {
28             ret.push_back(root->val);
29             inorderTraversalHelper(root->left, ret);
30             inorderTraversalHelper(root->right, ret);
31         }
32     }
33 };

方法二：迭代实现

 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 public:
12     vector<int> preorderTraversal(TreeNode* root) 
13     {
14         vector<int> ret;
15 
16         if (NULL == root)
17         {
18             return ret;
19         }
20         
21         stack<TreeNode *> st;
22         st.push(root);
23 
24         while (!st.empty())
25         {
26             TreeNode *tmp = st.top();
27             st.pop();
28             ret.push_back(tmp->val);
29 　　　　　　　 // 注意这里，栈是先进后出哦
30             if (tmp->right != NULL)
31             {
32                 st.push(tmp->right);
33             }
34             
35             if (tmp->left != NULL)
36             {
37                 st.push(tmp->left);
38             }
39         }
40 
41         return ret;
42     }
43 
44 };
45