LeetCode 144. 二叉树的前序遍历

class Solution {
public:
    
    vector preorderTraversal(TreeNode* root) {
       stack stk;
       vector ans;
       while(root||stk.size()){
            while(root){
                ans.push_back(root->val);
                stk.push(root);
                root=root->left;
           }
           root=stk.top()->right;
           stk.pop();
       }
       return ans;
    }
   
};

class Solution {
public:
    vector ans;
    vector preorderTraversal(TreeNode* root) {
        dfs(root);
        return ans;
    }
    void dfs(TreeNode* root){
        if(!root) return ;
        ans.push_back(root->val);
        dfs(root->left);
        dfs(root->right);
    }
};