leetcode 109 Convert Sorted List to Binary Search Tree

题目连接

https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/

Convert Sorted List to Binary Search Tree

Description

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
	TreeNode* sortedListToBST(ListNode* head) {
		if (!head) return NULL;
		vector<int> tmp; tmp.push_back(0);
		while (head) {
			tmp.push_back(head->val);
			head = head->next;
		}	
		function<TreeNode*(int, int)> dfs = [&dfs, &tmp](int lb, int ub) -> TreeNode* {
			if (lb > ub) return NULL;
			int m = (lb + ub) >> 1;
			auto p = new TreeNode(tmp[m]);
			p->left = dfs(lb, m - 1);
			p->right = dfs(m + 1, ub);
			return p;
		};
		return dfs(1, tmp.size() - 1);
	}
};