109. Convert Sorted List to Binary Search Tree.cpp

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

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5
*/

/**
 * 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> nums;
        while(head)
        {
            nums.emplace_back(head->val);
            head=head->next;
        }
        return DFS(nums,0,nums.size()-1);;
    }
    
    TreeNode* DFS(vector<int>& nums,int l,int r)
    {
        if(l>r) return NULL;
        int mid=(l+r)/2;
        TreeNode* root=new TreeNode(nums[mid]);
        root->left=DFS(nums,l,mid-1);
        root->right=DFS(nums,mid+1,r);
        return root;
    }
};