题解 | 二叉树的前序遍历

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    void preorder_helper(TreeNode *node, vector<int>& res)
    {
        if (node == nullptr) return;
        res.push_back(node->val);
        if (node->left) {
            preorder_helper(node->left, res);
        }
        if (node->right) {
            preorder_helper(node->right, res);
        }
    }
    /**
     * 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
        vector<int> res;
        preorder_helper(root, res);
        return res;
    }
};