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leet code

좀 대충 풀었는데, 바로 답이 나와서 패스했습니다.

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> retn;
        return po(root, retn);
    }

    vector<int>& po(TreeNode* root, vector<int>& rslt)
    {
        if(!root) return rslt;

        rslt.push_back(root->val);
        // 왼
        if(root->left)
            po(root->left, rslt);
        // 오
        if(root->right)
            po(root->right, rslt);

        return rslt;
    }
};