Recursive traversal of 414 binary tree

1、 Binary tree recursive traversal

Three elements of recursive algorithm ：

1. Determine the parameters and return values of recursive functions ： Determine which parameters need to be processed in the recursive process , Then add this parameter to the recursive function , What is the return value of each recursion, and then determine the return type of recursive function .
2. Determine termination conditions ： The recursive algorithm is finished , When it's running , We often encounter stack overflow errors , The termination condition is not written or the termination condition is not written correctly , The operating system also uses a stack structure to store the recursive information of each layer , If recursion doesn't stop , The memory stack of the operating system is bound to overflow .
3. Determine termination conditions ： The recursive algorithm is finished , When it's running , We often encounter stack overflow errors , The termination condition is not written or the termination condition is not written correctly , The operating system also uses a stack structure to store the recursive information of each layer , If recursion doesn't stop , The memory stack of the operating system is bound to overflow .

The former sequence traversal ：

class Solution {
    
public:
    void traversal(TreeNode* cur, vector<int>& vec) {
    
        if (cur == NULL) return;
        vec.push_back(cur->val);    //  in 
        traversal(cur->left, vec);  //  Left 
        traversal(cur->right, vec); //  Right 
    }
    vector<int> preorderTraversal(TreeNode* root) {
    
        vector<int> result;
        traversal(root, result);
        return result;
    }
};

In the sequence traversal ：

void traversal(TreeNode* cur, vector<int>& vec) {
    
    if (cur == NULL) return;
    traversal(cur->left, vec);  //  Left 
    vec.push_back(cur->val);    //  in 
    traversal(cur->right, vec); //  Right 
}

After the sequence traversal ：

void traversal(TreeNode* cur, vector<int>& vec) {
    
    if (cur == NULL) return;
    traversal(cur->left, vec);  //  Left 
    traversal(cur->right, vec); //  Right 
    vec.push_back(cur->val);    //  in 
}