Leetcode109. 有序链表转换二叉搜索树

题目传送： ​​https://leetcode.cn/problems/convert-sorted-list-to-binary-search-tree/​​

运行效率

代码如下：

class Solution {
      public TreeNode sortedListToBST(ListNode head) {
      //1、先想办法把链表里面的数 存储到数组中
        List<Integer> list = new ArrayList<>();
        while (head!=null){
            list.add(head.val);
            head=head.next;
        }
        Integer[] array = list.toArray(new Integer[0]);
        //2、然后再通过递归的方式来解决问题
        TreeNode helper = helper(array, 0, array.length - 1);
        return  helper;
    }


    public TreeNode helper(Integer[] nums,int start,int end){
        //处理边界情况
        if(start>end){
            return  null;
        }
        if(start==end){
            TreeNode treeNode = new TreeNode(nums[start]);
            return treeNode;
        }
        //中间节点的位置
        int mid =(start+end)/2;
        //以中间位置的数 生成根节点
        TreeNode root = new TreeNode(nums[mid]);
        //递归构建左子树
        TreeNode leftChild = helper(nums, start, mid-1);  
        //递归构建右子树
        TreeNode rightChild = helper(nums, mid + 1, end);
        root.left=leftChild;
        root.right=rightChild;
        return  root;
    }
}