LeetCode 988. 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 1 and 8500.
Each node in the tree will have a value between 0 and 25.

实现思路：

用0~25代表26个字母，然后从叶节点到根节点的众多条路径中，输出一条字母序最小的的一条路径，主要要注意的就是int转char。

AC代码：

class Solution {
		string ans;
	public:
		void dfs(TreeNode *root,string str) {
			if(!root) return;
			str+=char('a'+root->val);//int转char的技巧
			dfs(root->left,str);
			dfs(root->right,str);
			if(!root->left&&!root->right) {
				reverse(str.begin(),str.end());//因为是从下往上的字符串所以得倒过来
				if(str<ans) ans=str;
			}
		}
		string smallestFromLeaf(TreeNode* root) {
			for(int i=0; i<10000; i++) ans+='z';
			dfs(root,"");
			return ans;
		}
};