Convert Sorted List to Binary Search Tree——将链表转换为平衡二叉搜索树 &&convert-sorted-array-to-binary-search-tree—

Convert Sorted List to Binary Search Tree

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

为了满足平衡要求，容易想到提出中间节点作为树根，因为已排序，所以左右两侧天然满足BST的要求。

左右子串分别递归下去，上层根节点连接下层根节点即可完成。

递归找中点，然后断开前后两段链表，并继续找中点

 /**

  * 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 *sortedListToBST(ListNode *head) {

         if(head==NULL) return NULL;

         if(head->next==NULL) return new TreeNode(head->val);

         ListNode *mid=findMid(head);

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

         root->left=sortedListToBST(head);

         root->right=sortedListToBST(mid->next);

         return root;

     }

     ListNode *findMid(ListNode *root){

         if(root==NULL) return NULL;

         if(root->next==NULL) return root;

         ListNode *fast,*slow,*pre;

         fast=root;

         slow=root;

         while(fast!=NULL){

             fast=fast->next;

             if(fast!=NULL){

                 fast=fast->next;

                 pre=slow;

                 slow=slow->next;

             }

         }

         pre->next=NULL;

         return slow;

     }

 };

convert-sorted-array-to-binary-search-tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

 /**

  * 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 *sortedArrayToBST(vector<int> &num) {

         int n=num.size();

         if(n<) return NULL;

         return findMid(num,,n-);

     }

     TreeNode *findMid(vector<int> &num, int l,int r){

         if(l>r) return NULL;

         int mid=(l+r+)/;

         TreeNode *root=new TreeNode(num[mid]);

         root->left=findMid(num,l,mid-);

         root->right=findMid(num,mid+,r);

         return root;

     }

 };

巴特西

Convert Sorted List to Binary Search Tree——将链表转换为平衡二叉搜索树 &&convert-sorted-array-to-binary-search-tree——将数列转换为bst

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