LC.P109[有序链表转换二叉搜索树]

方法一：DFS

由于构造出的二叉搜索树的中序遍历结果就是链表本身，因此可以一边对链表遍历，一边对二叉树进行中序遍历。

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    ListNode head;

    public TreeNode sortedListToBST(ListNode head) {
        this.head = head;
        int n = 0;
        ListNode p = head;
        while (p != null) {
            p = p.next;
            ++n;
        }
        return build(0, n -1);
    }

    private TreeNode build(int left, int right) {
        if (left > right) return null;
        int mid = left + right >> 1;
        TreeNode node = new TreeNode();
        node.left = build(left, mid - 1);
        node.val = head.val;
        head = head.next;
        node.right = build(mid + 1, right);
        return node;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(logn)$