988. Smallest String Starting From Leaf

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988. Smallest String Starting From Leaf

Description

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

Hints/Notes

  • traverse the tree

Solution

Language: C++

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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 {
public:
    string res;

    string smallestFromLeaf(TreeNode* root) {
        traverse(root, "");
        return res;
    }

    void traverse(TreeNode* root, string cur) {
        if (!root) {
            return;
        }
        cur = char(root->val + 'a') + cur;
        if (!root->left && !root->right) {
            if (res == "" || res > cur) {
                res = cur;
            }
        }
        if (root->left) traverse(root->left, cur);
        if (root->right) traverse(root->right, cur);
    }
};