[leetcode_144]Binary Tree Preorder Traversal

Ugh. I got AC using recursion but it didn’t count as accepted – the problem wasn’t checked off. As someone with a bit of OCD, that’s painful. I’ll think of another approach later.

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class Solution {  
public:  
    vector<int> ans;  
    vector<int> preorderTraversal(TreeNode *root) {  
        // IMPORTANT: Please reset any member data you declared, as  
        // the same Solution instance will be reused for each test case.  
        ans.clear();  
        fun(root);  
        return ans;  
    }  
    void fun(TreeNode *root)  {  
        if (root != NULL) {  
            ans.push_back(root->val);  
            if (root->left != NULL)  
                fun(root->left);  
            if (root->right != NULL)  
                fun(root->right);  
        }  
    }  
};