Smallest String Starting From Leaf
Given theroot
of a binary tree, each node has a value from0
to25
representing the letters'a'
to'z'
: a value of0
represents'a'
, a value of1
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
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
前序遍历,注意一下退出的条件
Code
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public String smallestFromLeaf(TreeNode root) {
if (root == null) {
return null;
}
char ch = (char)('a' + root.val);
String left = smallestFromLeaf(root.left);
String right = smallestFromLeaf(root.right);
if (left == null && right == null) { // leaf
return "" + ch;
}
if (left == null) {
return right + ch;
}
if (right == null) {
return left + ch;
}
if (left.compareTo(right) < 0) {
return left + ch;
} else {
return right + ch;
}
}
}
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