144. Binary Tree Preorder Traversal

1.問題

• 給予一個binary tree, 回傳pre-order traversal得到的值

2.想法

• pre-order traversal

3.程式碼

/**
 * 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> preorderTraversal(TreeNode* root) {
        vector<int> v;
        preOrder(v, root);
        return v;
    }
private:
    void preOrder(vector<int>& v, TreeNode* root) {
        if (!root) {
            return;
        }
        v.push_back(root->val);
        preOrder(v, root->left);
        preOrder(v, root->right);
    }
};

4.Performance