LeetCode: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.

Solution:

/**
* 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) {
int len = 0;
ListNode *p = head;
while(p){
len++;
p = p->next;
}
return sortedListToBST(head,0,len-1);
}
private:
TreeNode *sortedListToBST(ListNode*& head,int l,int h){
if(l > h) return NULL;
int mid = l + (h-l)/2;
TreeNode *lChiled = sortedListToBST(head,l,mid-1);
TreeNode *p = new TreeNode(head->val);
p->left = lChiled;
head = head->next;
p->right = sortedListToBST(head,mid+1,h);
return p;
}
};

这种bottom-up 方法很巧妙

Time complexity:O(N)
http://leetcode.com/2010/11/convert-sorted-list-to-balanced-binary.html