Convert Sorted List to Binary Search Tree

Convert Sorted List to Binary Search TreeOct 3 '123397
/ 9589

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

 

剛開始想用課本上的構造平衡二叉樹的方法，當不滿足時，回溯，旋轉，結果在運行大的資料時，逾時。

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; *//** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void insertBST(TreeNode* &root, int val)    {        if(root == NULL)        {            root = new TreeNode(val);        }        else if(val > root->val)        {            insertBST(root->right, val);        }        else if(val < root->val)        {            insertBST(root->left, val);        }    }    void turnLeft(TreeNode* &root)    {                TreeNode* tempCR = root->right->left;        root->right->left = root;                root = root->right;        root->left->right = tempCR;    }    int adjust(TreeNode* &root)    {        if(root)        {            int hl = adjust(root->left);            int hr = adjust(root->right);            if(hr > hl + 1)            {                turnLeft(root);                return max(hl, hr);            }            return max(hl, hr)+1;        }        return 0;    }    TreeNode *sortedListToBST(ListNode *head) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(head == NULL)            return NULL;        TreeNode* root = NULL;        while(head)        {            insertBST(root, head->val);            adjust(root);            head = head->next;        }        return root;    }};

 

後來，想直接在中間節點分開，遞迴構造。複雜度應該為nlogn

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; *//** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode* buildTree(ListNode* head, int len)    {        if(head && len > 0)        {            int l = len/2;            ListNode* p = head;            int count = 0;            while(p && count < l)            {                count ++;                p = p->next;            }            TreeNode* tre = new TreeNode(p->val);            TreeNode* lc = buildTree(head, count);            TreeNode* lr = buildTree(p->next, len - count -1);            tre->left = lc;            tre->right = lr;            return tre;        }        return NULL;    }    TreeNode *sortedListToBST(ListNode *head) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        int len = 0;        ListNode* p = head;        while(p)        {            len ++;            p = p ->next;        }        return buildTree(head,len);    }};