109. Convert Sorted List to Binary Search Tree

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109. Convert Sorted List to Binary Search Tree

题目

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

解析

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode *sortedListToBST(ListNode *head) {

ListNode* slow = head;
ListNode* fast = head;
ListNode* pre = NULL;
if (!head)
{
return NULL;
}
if (!head->next)
{
TreeNode* temp = new TreeNode(head->val);
return temp;
}
while (fast&&fast->next)
{
pre = slow;
slow = slow->next;
fast = fast->next->next;
}

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

pre->next = NULL;
slow = slow->next;
root->left = sortedListToBST(head);
root->right = sortedListToBST(slow);

return root;
}
};

题目来源