【leetcode】Convert Sorted List to Binary Search Tree

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.

 

 

可以采用类似于Covert Sorted Array to Binary Search Tree的方法，但是寻找中点对于链表来说效率较低

可以采用更高效的递归方式，无需寻找中点

注意引用传递：TreeNode *buildTree(ListNode *&node,int left,int right)

 

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