【Leetcode】Convert Sorted List to Binary Search Tree

最新推荐文章于 2025-09-10 22:03:14 发布

yyz20002008

最新推荐文章于 2025-09-10 22:03:14 发布

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文章标签： leetcode java arraylist ListNode Binary Search Tree

本文详细介绍了如何将一个已排序的单链表转换为一个高度平衡的二叉搜索树，包括使用动态数组和快慢指针的方法。

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

Java:

http://blog.youkuaiyun.com/u013027996/article/details/21323449

1. 链表转换成动态数组

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null){
            return null;
        }
        int len = 0;
        List<Integer> list = new ArrayList<Integer>();
        while(head != null){
            list.add(head.val);
            head = head.next;
            len++;
        }
        return createBSTree(list, 0, len - 1);
    }
	private TreeNode createBSTree(List<Integer> list, int low, int high) {
		if (low > high) {
			return null;
		}
		if (low == high) {
			return new TreeNode(list.get(low));
		}
		int mid = (low + high) >> 1;
		TreeNode root = new TreeNode(list.get(mid));
		root.left = createBSTree(list, low, mid - 1);
		root.right = createBSTree(list, mid + 1, high);
		return root;
	}
}

2.快慢指针

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null){
            return null;
        }
        return createBSTree(head, null);
    }
	private TreeNode createBSTree(ListNode start, ListNode end) {
		if(start == end){
		    return null;
		}
		ListNode fastNode = start;
		ListNode slowNode = start;
		while(fastNode != end && fastNode.next != end){
		    slowNode = slowNode.next;
		    fastNode = fastNode.next.next;
		}
		TreeNode root = new TreeNode(slowNode.val);
		root.left = createBSTree(start, slowNode);
		root.right = createBSTree(slowNode.next, end);
		return root;
	}
}