LC 988. Smallest String Starting From Leaf

Given the root of a binary tree, each node has a value from 0 to 25 representing the letters 'a' to 'z': a value of 0 represents 'a', a value of 1represents 'b', and so on.

Find the lexicographically smallest string that starts at a leaf of this tree and ends at the root.

(As a reminder, any shorter prefix of a string is lexicographically smaller: for example, "ab" is lexicographically smaller than "aba".  A leaf of a node is a node that has no children.)

Runtime:  4 ms, faster than 100.00% of C++ online submissions for Smallest String Starting From Leaf.

Memory Usage:  884.7 KB, less than 100.00% of C++ online submissions for Smallest String Starting From Leaf.

class Solution {
public:
  string smallestFromLeaf(TreeNode* root) {
    vector<string> a;
    helper(root, a, "");
    sort(a.begin(), a.end());
    return a[0];
  }
  void helper(TreeNode* root, vector<string>& a, string parent){
    if(!root) return;
    string tmpc(1,(char)('a'+root->val));
    string tmps = tmpc + parent;
    if(!root->left && !root->right){
      a.push_back(tmps);
      return;
    }
    helper(root->left, a, tmps);
    helper(root->right, a, tmps);
  }
};