Convert Sorted List to Binary Search Tree

Posted 2021-12-19 tianzeng

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

code

class Solution

public:
    TreeNode* sortedListToBST(ListNode* head)
    
        if(head==nullptr)
            return nullptr;

        vector<int> res;
        auto tmp=head;
        while(tmp)
        
            res.emplace_back(tmp->val);
            tmp=tmp->next;
        
        return dfs(0,res.size()-1,res);
    
private:
    TreeNode *dfs(int start,int end,vector<int>& res)
    
        if(end<start)
            return nullptr;

        int index=(start+end)/2;
        TreeNode *root=new TreeNode(res.at(index));
        root->left=dfs(start,index-1,res);
        root->right=dfs(index+1,end,res);

        return root;
    
;