递归算法的调试

package c;

class ListNode {
	int val;
	ListNode next;
	
	ListNode(int x) {
		val = x;
	}
	
	// 链表结点的构造函数
	// 使用arr为参数, 创建一个链表, 当前的ListNode为链表头结点
	ListNode(int [] arr) {
		
		if(arr == null || arr.length == 0) {
			throw new IllegalArgumentException("arr can not be empty");
		}
		
		this.val = arr[0];
		ListNode cur = this;
		for(int i = 1; i < arr.length; ++ i) {
			cur.next = new ListNode(arr[i]);
			cur = cur.next;
		}
	}
	
	// 以当前节点为头结点的链表信息字符串
	@Override
	public String toString() {
		
		StringBuilder s = new StringBuilder();
		ListNode cur = this;
		while(cur != null) {
			s.append(cur.val + "->");
			cur = cur.next;
		}
		s.append("NULL");
		return s.toString();
	}
}

public class Solution {
	
	public ListNode removeElements(ListNode head, int val, int depth) {
		
		String depthString = generateDepthString(depth);
		
		System.out.print(depthString);
		System.out.println("Call : remove " + val + " in " + head);
		
		if(head == null) {
			System.out.print(depthString);
			System.out.println("Return : " + head);
			return head;
		}
		
		ListNode res = removeElements(head.next, val, depth + 1);
		System.out.print(depthString);
		System.out.println("After remove " + val + ": " + res);
		
		ListNode ret;
		if(head.val == val) {
			ret = res;
		}
		else {
			head.next = res;
			ret = head;
		}
		
		System.out.print(depthString);
		System.out.println("Return : " + ret);
		return ret;
	} 
	
	private String generateDepthString(int depth) {
		StringBuilder res = new StringBuilder();
		for(int i = 0; i < depth; ++ i) {
			res.append("--");
		}
		return res.toString();
	}
	
	public static void main(String[] args) {
		
		int[] nums = {1,2,6,3,4,5,6};
		ListNode head = new ListNode(nums);
		System.out.println(head);
		
		ListNode res = (new Solution()).removeElements(head, 6, 0);
		System.out.println(res);
	}
}