109.convert-sorted-list-to-binary-search-tree.cpp

/*
 * @lc app=leetcode id=109 lang=cpp
 *
 * [109] Convert Sorted List to Binary Search Tree
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

struct ListNode {
     int val;
     ListNode *next;
     ListNode() : val(0), next(nullptr) {}
     ListNode(int x) : val(x), next(nullptr) {}
     ListNode(int x, ListNode *next) : val(x), next(next) {}
 };

struct TreeNode {
     int val;
     TreeNode *left;
     TreeNode *right;
     TreeNode() : val(0), left(nullptr), right(nullptr) {}
     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 };

class Solution {
public:
    TreeNode* sortedListToBST(ListNode* head) {
        return buildTree(head, nullptr);
    }

    TreeNode* buildTree(ListNode* left, ListNode* right) {
        if(left == right){
            return nullptr;
        }
        
        ListNode* mid = getMid(left, right);

        TreeNode* root = new TreeNode(mid->val);

        root->left = buildTree(left, mid);
        root->right = buildTree(mid->next, right);
        return root;

    }
    ListNode* getMid(ListNode* left, ListNode*right) {
        ListNode* fast = left;
        ListNode* slow = left;
        while(fast != right && fast->next != right){
            fast = fast->next;
            fast = fast->next;
            slow = slow->next;
        }
        return slow;
    }
    
};
// @lc code=end