109. 有序链表转换二叉搜索树

109. 有序链表转换二叉搜索树

/**
 * 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 {
    public TreeNode sortedListToBST(ListNode head) {
        if (head == null)
            return null;
        return dfs(head, null);
    }
    private TreeNode dfs(ListNode left, ListNode right) {
        if (left == right)
            return null;
        ListNode slow = left;
        ListNode fast = left;
        while (fast != right && fast.next != right) {
            fast = fast.next.next;
            slow = slow.next;
        }
        TreeNode root = new TreeNode(slow.val);
        // 这里是dfs(left, slow)是因为区间是[left, right)
        root.left = dfs(left, slow);
        root.right = dfs(slow.next, right);
        return root;
    }
}