Smallest String Starting From Leaf Problem


Description

LeetCode Problem 988.

You are given the root of a binary tree where each node has a value in the range [0, 25] representing the letters ‘a’ to ‘z’.

Return 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.

Example 1:

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Input: root = [0,1,2,3,4,3,4]
Output: "dba"

Example 2:

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Input: root = [25,1,3,1,3,0,2]
Output: "adz"

Example 3:

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Input: root = [2,2,1,null,1,0,null,0]
Output: "abc"

Constraints:

  • The number of nodes in the tree is in the range [1, 8500].
  • 0 <= Node.val <= 25


Sample C++ Code

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    string ans;
public:
    void helper(TreeNode *root,string &s) {
        if (!root) return;
        s.push_back(root->val + 'a');
        if (root->left == root->right) {
            string str = s;
            reverse(str.begin(), str.end());
            if (ans.empty() || str < ans)
                ans = str;
        }
        helper(root->left, s);
        helper(root->right, s);
        s.pop_back();
    }
    
    string smallestFromLeaf(TreeNode* root) {
        string s = "";
        helper(root, s);
        return ans;
    }
};




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