树的前序遍历

题目链接

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
vector<int> res;
    
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型vector
     */
    vector<int> preorderTraversal(TreeNode* root) {
        // write code here
        if(root==NULL){
            return {};
        }
        
        res.push_back(root->val);
        preorderTraversal(root->left);
        preorderTraversal(root->right);
        
        return res;
    }
};