LeetCode: Convert Sorted List to Binary Search Tree

Problem:

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

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Solution:二分查找的运用

/**
 * 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) {
        
        int length=calLength(head);
        return createBST(0,length-1,head);
        
        
        
    }
    //二分构建二叉查找树
    TreeNode* createBST(int left,int right,ListNode *p)
    {
        if(right<left) return NULL;
        
        int middle=(left+right)/2;
        
        ListNode *node=p;
        for(int i=left;i<middle;i++)
            node=node->next;
        TreeNode *root=new TreeNode(node->val);
        TreeNode *leftnode=createBST(left,middle-1,p);
        TreeNode *rightnode=createBST(middle+1,right,node->next);
        root->left=leftnode;
        root->right=rightnode;
        return root;
        
    }
    
    //计算链表长度
    int calLength(ListNode *p)
    {
        int len=0;
        while(p)
        {
            len++;
            p=p->next;
        }
        return len;
    }
    
};