leetcode 109 Convert Sorted List to Binary Search Tree

最新推荐文章于 2021-06-18 20:00:15 发布

walter1990

最新推荐文章于 2021-06-18 20:00:15 发布

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本文介绍了一种将有序单链表转换为高度平衡的二叉搜索树的方法。通过递归地构建二叉树，确保了树的高度平衡，并保持了节点值的排序特性。

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

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/**
 * 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 *buildTree(ListNode *&head, int left, int right) {
		if(left > right) {
			return NULL;
		}
		int mid = left+(right-left+1)/2;
		TreeNode *leftNode = buildTree(head, left, mid-1);

		TreeNode *root = new TreeNode(head->val);
		head = head->next;
		root->left = leftNode;

		TreeNode *rightNode = buildTree(head, mid+1, right);

		root->right=rightNode;
		return root;
	}
	TreeNode *sortedListToBST(ListNode *head) {
		TreeNode *root=NULL;
		if(head==NULL) return root;

		ListNode *l = head;
		int cnt = 0;

		while(l!=NULL) {
			cnt++;
			l=l->next;
		}
		
		root = buildTree(head, 0, cnt-1);

		return root;
	}
};