109. Convert Sorted List to Binary Search Tree

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problem

solution

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class Solution {
public:
    TreeNode* sortedListToBST(ListNode* head) {
        if(!head) return nullptr;
        if(!head->next) return new TreeNode(head->val);
        ListNode *slow = head, *fast = head, *pre = head;
        while(fast && fast->next){
            pre = slow;
            fast =fast->next->next;
            slow = slow->next;
        }
        pre->next = nullptr;
        TreeNode *root = new TreeNode(slow->val);
        root->left = sortedListToBST(head);
        if(slow->next) root->right = sortedListToBST(slow->next);
        return root;
    }
};

analysis

  • time complexity O(n)
  • space complexity O(n)