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/**
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 * Definition for a binary tree node.
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 * struct TreeNode {
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 *     int val;
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 *     TreeNode *left;
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 *     TreeNode *right;
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 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
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 * right(right) {}
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 * };
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 */
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class Solution {
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 public:
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  string res;
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  void solve(TreeNode *root, string cur) {
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    if (!root) return;
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    cur.push_back((char)('a' + root->val)); // converting integer to corresponding characer
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    if (!root->left and !root->right) {
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      // reversing the string since it is computed from root to leaf, but we
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      // need viceversa
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      reverse(cur.begin(), cur.end());
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      if (res == "" or cur < res) res = cur; // updating the result based on lexicographical order
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      return;
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    }
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    solve(root->left, cur);
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    solve(root->right, cur);
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    return;
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  }
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  string smallestFromLeaf(TreeNode *root) {
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    if (!root) return "";
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    solve(root, "");
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    return res;
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  }
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};