109. Convert Sorted List to Binary Search Tree(快慢指針、遞歸、BST)

package Depth_first_Search;

public class SortedListToBST_109 {

	public class ListNode {
		int val;
		ListNode next;

		ListNode(int x) {
			val = x;
		}
	}

	public class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

		TreeNode(int x) {
			val = x;
		}
	}

	public TreeNode sortedListToBST(ListNode head) {
		if(head==null) {
			return null;
		}
		
		ListNode mid=findMiddleNode(head);
		TreeNode node=new TreeNode(mid.val);
		// 遞歸結束標誌
		if(head==mid) {
			return node;
		}
		node.left=sortedListToBST(head);
		node.right=sortedListToBST(mid.next);
		return node;

	}
	ListNode findMiddleNode(ListNode head){
		ListNode prePtr=null;
		ListNode slowPtr=head;
		ListNode fastPtr=head;
		while(fastPtr!=null&&fastPtr.next!=null) {
			prePtr=slowPtr;
			slowPtr=slowPtr.next;
			fastPtr=fastPtr.next.next;
		}
		// 找到鏈表中的中間元素後，將鏈表從中間元素的左側斷開
		if(prePtr!=null) {
			prePtr.next=null;
		}
		return slowPtr;
	}

	
}