经典算法前序遍历二叉树

1、题目描述
给定一个二叉树，返回它的前序遍历。

示例:

输入: [1,null,2,3]
1

2
/
3

输出: [1,2,3]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

2、我的代码
/**

Definition for a binary tree node.
struct TreeNode {
```
int val;
```
```
TreeNode *left;
```
```
TreeNode *right;
```

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

};
/
class Solution {
public:
vector preorderTraversal(TreeNode root) {
TreeNode* cur_node;
if(root == NULL)
return result_vec;
```
 result_vec.push_back(root->val);
 preorderTraversal(root->left);
 preorderTraversal(root->right);

 return result_vec;
```
}
vector result_vec;
};

使用stack
/**

Definition for a binary tree node.
struct TreeNode {
```
int val;
```
```
TreeNode *left;
```
```
TreeNode *right;
```

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

};
/
class Solution {
public:
vector preorderTraversal(TreeNode root) {
TreeNode* cur_node;
stack<TreeNode*> stack_node;
TreeNode* pre_node;
```
 if(root == NULL)
     return result_vec;
 cur_node = root;
 while(cur_node != NULL || !stack_node.empty()){
     if(cur_node == NULL){
         cur_node = stack_node.top();
         stack_node.pop();
     }
     result_vec.push_back(cur_node->val);
     if(cur_node->right != NULL)
         stack_node.push(cur_node->right);
     cur_node = cur_node->left;
 };

 return result_vec;
```
}
vector result_vec;
};
3、网上较好的解法
莫里斯算法逻辑并不清晰，但可能省存储空间
4、自己可以改进的地方
a、利用stack的方法，执行效率并不高，逻辑设计的不对
b、
5、优化代码至简无可简(效率击败：63%，内存击败：100%)
/**
Definition for a binary tree node.
struct TreeNode {
```
int val;
```
```
TreeNode *left;
```
```
TreeNode *right;
```

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

};
/
class Solution {
public:
vector preorderTraversal(TreeNode root) {
TreeNode* cur_node;
stack<TreeNode*> stack_node;
TreeNode* pre_node;

 if(root == NULL)
     return result_vec;
 cur_node = root;
 stack_node.push(cur_node);
 while(!stack_node.empty()){
     cur_node = stack_node.top();
     stack_node.pop();
     result_vec.push_back(cur_node->val);
     if(cur_node->right != NULL)
         stack_node.push(cur_node->right);
     if(cur_node->left != NULL)
         stack_node.push(cur_node->left);
 };

 return result_vec;

}
vector result_vec;
};
6、获得的思考
遍历顺序不一样时，入stack的逻辑是不同的，否则会影响逻辑清晰度甚至是执行效率

digitaluser

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经典算法前序遍历二叉树

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