LeetCode 144. 二叉树的前序遍历 Binary Tree Postorder Traversal (Medium)

/**
 * 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> ret;  //保存路径值
    vector<int> preorderTraversal(TreeNode* root) {
        
        if (root == nullptr) return ret;

        ret.push_back(root->val);
        preorderTraversal(root->left);
        preorderTraversal(root->right);

        return ret;
    }
};