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