109. Convert Sorted List to Binary Search Tree (List; Divide-and-Conquer, dfs)

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

struct ListNode {
   int val;
   ListNode *next;
   ListNode(int x) : val(x), next(NULL) {}
 };
struct TreeNode {
   int val;
   TreeNode *left;
   TreeNode *right;
   TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        if(!head) return NULL;
        int size = 1;
        ListNode * listNode = head->next;
        while(listNode)
        {
            size++;
            listNode = listNode->next;
        }
        TreeNode* root = new TreeNode(0);
        binarySearch(head,size,root);
        return root;
    }
    void binarySearch(ListNode *head,int size, TreeNode* treeNode) //二分法，对于Array，需要一头一尾两个指针；对于List需要头指针及size
    {
        if(size == 1)
        {
            treeNode->val = head->val;
            return;
        }
        ListNode * listNode = head;
        int mid = size>>1;
        for(int i = 1 ; i<mid;i++)
        {
            listNode = listNode ->next;
        }
        treeNode->val = listNode->next->val;
        ListNode* head2 = listNode->next->next;
        listNode->next = NULL;
        treeNode->left = new TreeNode(0); //先申请号空间，再传递。否则在被调用函数中指针本身值的变化并不会影响调用者
        binarySearch(head,mid,treeNode->left);
        if(head2)
        {
            treeNode->right = new TreeNode(0);
            binarySearch(head2,size-mid-1,treeNode->right);
        }
    }
};

也可以通过引用传递，这样就不需要先初始化。

class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        if(!head) return NULL;
        int size = 1;
        ListNode * listNode = head->next;
        while(listNode)
        {
            size++;
            listNode = listNode->next;
        }
        TreeNode* root = new TreeNode(0);
        binarySearch(head,size,root);
        return root;
    }
    void binarySearch(ListNode *head,int size, TreeNode* &treeNode) //按引用传递参数
    {
        if(size == 1)
        {
            treeNode=new TreeNode(head->val);
            return;
        }
        ListNode * listNode = head;
        int mid = size>>1;
        for(int i = 1 ; i<mid;i++)
        {
            listNode = listNode ->next;
        }
        treeNode = new TreeNode (listNode->next->val); //在被调用处申请空间
        ListNode* head2 = listNode->next->next;
        listNode->next = NULL;
        binarySearch(head,mid,treeNode->left); //引用的是0x0000的地址
        if(head2)
        {
            binarySearch(head2,size-mid-1,treeNode->right);
        }
    }
};

注意，NULL是一个宏定义

#define NULL 0

即NULL = (void*) 0