前序遍历二叉树
binary-tree-preorder-traversal
https://www.nowcoder.com/practice/501fb3ca49bb4474bf5fa87274e884b4?tpId=46&tqId=29036&tPage=1&rp=1&ru=%2Fta%2Fleetcode&qru=%2Fta%2Fleetcode%2Fquestion-ranking
利用递归简单的实现：
class Solution {
public:
vector<int> preorderTraversal(TreeNode *root) {
vector<int> res;
helper(root, res);
return res;
}
private:
void helper(TreeNode * root ,vector<int>& res) {
if(root == NULL)return ;
res.push_back(root->val);
helper(root->left , res);
helper(root->right, res);
}
};
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