144. Binary Tree Preorder Traversal

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problem

solution

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void preorder(TreeNode *root, vector<int>& ret){
        if(!root) return;
        
        ret.push_back(root->val);
        preorder(root->left, ret);
        preorder(root->right, ret);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int>  ret;
        preorder(root, ret);
        return ret;
        
    }
};