leetcode--Convert Sorted List to Binary Search Tree

最新推荐文章于 2021-11-02 12:48:41 发布

crazy__chen

最新推荐文章于 2021-11-02 12:48:41 发布

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本文介绍了一种将升序单链表转换为高度平衡的二叉搜索树的方法。通过先遍历链表计算长度，并将链表元素转换为数组，再递归地构建平衡二叉搜索树。

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
		ListNode t = head;
		int count = 0;
		while(t!=null){
			count++;
			t = t.next;
		}
		int[] nums = new int[count];
		t = head;
		int i=0;
		while(t!=null){
			nums[i++] = t.val;
			t = t.next;
		}
		return helper(nums, 0, nums.length-1);
    }
	
	public TreeNode helper(int[] nums,int start,int end){
		if(start>end) return null;
		if(start==end) return new TreeNode(nums[start]);
		int mid = start+(end-start)/2;
		TreeNode t = new TreeNode(nums[mid]);
		t.left = helper(nums, start, mid-1);
		t.right = helper(nums, mid+1, end);
		return t;
	}
}