988. Smallest String Starting From Leaf
     Medium
     1.6K
     227
     Companies
     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:

Input: root = [0,1,2,3,4,3,4]
Output: “dba”
Example 2:

Input: root = [25,1,3,1,3,0,2]
Output: “adz”
Example 3:

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

解法1：遍历二叉树即可。
注意这里是要找到顺序最小的字符串，所以gMaxPath要初始化成ascii很大的字符串。而ascii table里面，数字<大写字母<小写字母。在前128个ascii code里面，比’z’还大的只有’{‘,’}‘,‘|’和DEL。所以可以初始化成"{}"。

/*** 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 smallestFromLeaf(TreeNode* root) {helper(root, "");return gMaxPath;}
private:string gMaxPath = "{}";//'{' or '}' > 'z'void helper(TreeNode *root, string path) {if (!root) return;path += 'a' + root->val;if (!root->left && !root->right) {reverse(path.begin(), path.end());gMaxPath = min(gMaxPath, path);return;}helper(root->left, path);helper(root->right, path);return;}
};

二刷：path改成引用。
需要注意：

1. 如果不是引用，可以直接调用helper(root, “”)，但是如果是引用的话，需要先定义string path = “”, 然后调用helper(root, path)。
2. 在 if (!root->left && !root->right) {}里面return前也要 path.pop_back();

/*** 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 smallestFromLeaf(TreeNode* root) {string path = "";helper(root, path);return gMaxPath;}
private:string gMaxPath = "{}";//'{' or '}' > 'z'void helper(TreeNode *root, string &path) {if (!root) return;path += 'a' + root->val;if (!root->left && !root->right) {string tmp = path;reverse(tmp.begin(), tmp.end());gMaxPath = min(gMaxPath, tmp);path.pop_back();return;}helper(root->left, path);helper(root->right, path);path.pop_back();return;}
};