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