LeetCode:144.二叉树的前序遍历

/**
 * 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<int>obj;
    vector<int> preorderTraversal(TreeNode* root) {
        if(root==NULL)
        {
            return obj;
        }
        obj.push_back(root->val);
        if(root->left!=NULL)
        {
            preorderTraversal(root->left);
        }
        
        if(root->right!=NULL)
        {
            preorderTraversal(root->right);
        }
        return obj;
    }
};