Leetcode--Convert Sorted List to Binary Search Tree

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Problem Description:

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

分析：很容易想到的一种解法是将链表中所有的元素保存到数组中，然后每次取中间值进行构造，时间复杂度为O(n)，空间复杂度为O(n)。具体实现如下：

/**
 * 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* createTree(vector<ListNode *> &listvec,int begin,int last)
    {
        if(begin>last)
            return NULL;
        if(begin==last)
        {
            int val=listvec[begin]->val;
            TreeNode *head=new TreeNode(val);
            return head;
        }
        else
        {
            int mid=(begin+last)/2;
            int val=listvec[mid]->val;
            TreeNode *head=new TreeNode(val);
            head->left=createTree(listvec,begin,mid-1);
            head->right=createTree(listvec,mid+1,last);
            return head;
        }
    }


    TreeNode *sortedListToBST(ListNode *head) {
        TreeNode *Treehead=NULL;
        if(head==NULL)
            return Treehead;
        vector<ListNode *> listvec;
        ListNode *p=head;
        while(p)
        {
            listvec.push_back(p);
            p=p->next;
        }
        Treehead=createTree(listvec,0,listvec.size()-1);
        return Treehead;
    }
};

   TreeNode *sortedListToBST(ListNode *head) {
        int len = 0;
        ListNode * node = head;
        while (node != NULL)
        {
            node = node->next;
            len++;
        }
        return buildTree(head, 0,len-1);
    }
    
    TreeNode *buildTree(ListNode *&node, int start, int end)
    {
        if (start > end) return NULL;
        int mid = start + (end - start) / 2;
        TreeNode *left = buildTree(node, start, mid-1);
        TreeNode *root = new TreeNode(node->val);
        root->left = left;
        node = node->next;
        root->right = buildTree(node, mid+1, end);
        return root;
    }

Leetcode--Convert Sorted List to Binary Search Tree

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原文地址：http://blog.csdn.net/longhopefor/article/details/39082653