1 #define pb push_back
 2 #define _for(i,a,b) for(int i = (a);i < (b);i ++)
 3 class Solution
 4 {
 5     public:
 6         void preorder(vector<int> v,TreeNode* root,string &s)
 7         {
 8 
 9             if(root->left==NULL&&root->right==NULL)
10             {
 
11                 v.pb(root->val);
12                 string tmp;
13                 for(int i = v.size()-1;i >= 0;i --)
14                     tmp += v[i]+‘a‘;
15                 if(s.empty())
16                     s = tmp;
17                 else if(s > tmp)
18                     s = tmp;
 
19                 return ;
20             }
21             v.pb(root->val);
22             if(root->left)
23             preorder(v,root->left,s);
24             if(root->right)
25             preorder(v,root->right,s);
26         }
27         string smallestFromLeaf(TreeNode* root)
 
28         {
29             vector<int> v;
30             string s;
31             preorder(v,root,s);
32             return s;
33         }
34 };

Leetcode-988 (從葉結點開始的最小字符串)