leetcode -day19 Convert Sorted List to Binary Search Tree

1、

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.

分析：将一个升序排列的链表转换为平衡二叉搜索树，采用递归的方式，先找到链表的中点，作为二叉树的根，然后递归求解左右子树。

如下：

class Solution {
public:
TreeNode *sortedListToBST(ListNode *head) {
if(!head){
return NULL;
}
if(!head->next){
return new TreeNode(head->val);
}
ListNode* midNode = findMidNode(head);
TreeNode* root = new TreeNode(midNode->val);
TreeNode* leftSubTree = sortedListToBST(head);
TreeNode* rightSubTree = sortedListToBST(midNode->next);
if(leftSubTree){
root->left = leftSubTree;
}
if(rightSubTree){
root->right = rightSubTree;
}
return root;
}
ListNode* findMidNode(ListNode* head){
ListNode* node1 = head;
if(!node1->next){
return node1;
}
ListNode* node2 = head->next;
while(node2 && node2->next && node2->next->next){
node2 = node2->next;
node2 = node2->next;
node1 = node1->next;
}
ListNode* midNode = node1->next;
node1->next = NULL;
return midNode;
}
};