988. Smallest String Starting From Leaf
https://leetcode.com/problems/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 1 represents '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.)
Example 1:
Input: [0,1,2,3,4,3,4] Output: "dba"
Example 2:
Input: [25,1,3,1,3,0,2] Output: "adz"
Example 3:
Input: [2,2,1,null,1,0,null,0] Output: "abc"
Note:
- The number of nodes in the given tree will be between
1and8500. - Each node in the tree will have a value between
0and25.
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| /** | |
| * Definition for a binary tree node. | |
| * public class TreeNode { | |
| * int val; | |
| * TreeNode left; | |
| * TreeNode right; | |
| * TreeNode() {} | |
| * TreeNode(int val) { this.val = val; } | |
| * TreeNode(int val, TreeNode left, TreeNode right) { | |
| * this.val = val; | |
| * this.left = left; | |
| * this.right = right; | |
| * } | |
| * } | |
| */ | |
| class Solution { | |
| String ans = ""; | |
| public String smallestFromLeaf(TreeNode root) { | |
| if (root == null) { | |
| return ""; | |
| } | |
| dfs(root, ""); | |
| return ans; | |
| } | |
| private void dfs(TreeNode node, String prefix) { | |
| if (node == null) { | |
| return ; | |
| } | |
| prefix = String.valueOf((char)('a' + node.val)) + prefix; | |
| if (node.left == null && node.right == null) { | |
| if (ans.equals("") || prefix.compareTo(ans) < 0) { | |
| ans = prefix; | |
| } | |
| return ; | |
| } | |
| dfs(node.left, prefix); | |
| dfs(node.right, prefix); | |
| } | |
| } |
