如何判断链表是否有环？
准备两个指针，快指针每次走两步，慢指针每次走一步。
如果两个指针某一次相遇则一定有环
如何找到入环的第一个节点？（图中的6）
当快慢指针相遇后，让快指针回到起点每次走一步，慢指针继续走。
直到两个指针相遇则为入环的第一个节点

// 找到链表第一个入环节点，如果无环，返回null
	public static Node getLoopNode(Node head) {
		if (head == null || head.next == null || head.next.next == null) 									{  //只有1,2,0个节点必定不相交
			return null;
		}
		// n1 慢  n2 快
		Node slow = head.next; // n1 -> slow
		Node fast = head.next.next; // n2 -> fast
		while (slow != fast) {
			if (fast.next == null || fast.next.next == null) {
				return null;//如果快指针为空，则必定不相交
			}
			fast = fast.next.next;
			slow = slow.next;
		}
		// slow fast  相遇
		fast = head; // n2 -> walk again from head
		while (slow != fast) {
			slow = slow.next;
			fast = fast.next;
		}
		return slow;
	}

/// 如果两个链表都无环，返回第一个相交节点，如果不想交，返回null
	public static Node noLoop(Node head1, Node head2) {
		if (head1 == null || head2 == null) {
			return null;
		}
		Node cur1 = head1;
		Node cur2 = head2;
		int n = 0;
		while (cur1.next != null) {
			n++;
			cur1 = cur1.next;
		}
		while (cur2.next != null) {
			n--;
			cur2 = cur2.next;
		}
		if (cur1 != cur2) {
			return null;
		}
		// n  :  链表1长度减去链表2长度的值
		cur1 = n > 0 ? head1 : head2; // 谁长，谁的头变成cur1
		cur2 = cur1 == head1 ? head2 : head1; // 谁短，谁的头变成cur2
		n = Math.abs(n);
		while (n != 0) {
			n--;
			cur1 = cur1.next;
		}
		while (cur1 != cur2) {
			cur1 = cur1.next;
			cur2 = cur2.next;
		}
		return cur1;
	}

两个无环链表如何找到第一个相交的节点？
先判断两个链表最后一个节点是不是公共部分，如果不是说明两个链表是平行的，就不需要再判断了。
如果尾部想等之后，n为链表节点的差值。先让长链表走差值步，然后两个链表的指针开始往前跑，直到相交

链表相交的情况
1.两个链表都是无环链表
2.两个链表都是有环链表
（如果一个链表有环，一个链表无环不可能相交）

两个有环链表的三种情况
当第二种情况，把入环的第一个节点当作最后一个节点（这样就不用看环了）就可以按照两个无环的情况求得相交的第一个节点

当loop1不等于loop2时为第一种和第三种情况
当loop1往前走遇到loop2了为第三种情况，此时返回loop1和loop2，两个都是第一个相交的节点。
没有遇到为第一种情况，直接返回空

// 两个有环链表，返回第一个相交节点，如果不想交返回null
	public static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {
		Node cur1 = null;
		Node cur2 = null;
		if (loop1 == loop2) {
			cur1 = head1;
			cur2 = head2;
			int n = 0;
			while (cur1 != loop1) {
				n++;
				cur1 = cur1.next;
			}
			while (cur2 != loop2) {
				n--;
				cur2 = cur2.next;
			}
			cur1 = n > 0 ? head1 : head2;
			cur2 = cur1 == head1 ? head2 : head1;
			n = Math.abs(n);
			while (n != 0) {
				n--;
				cur1 = cur1.next;
			}
			while (cur1 != cur2) {
				cur1 = cur1.next;
				cur2 = cur2.next;
			}
			return cur1;
		} else {
			cur1 = loop1.next;
			while (cur1 != loop1) {
				if (cur1 == loop2) {
					return loop1;
				}
				cur1 = cur1.next;
			}
			return null;
		}
	}

public class Code01_FindFirstIntersectNode {

	public static class Node {
		public int value;
		public Node next;

		public Node(int data) {
			this.value = data;
		}
	}

	public static Node getIntersectNode(Node head1, Node head2) {
		if (head1 == null || head2 == null) {
			return null;
		}
		Node loop1 = getLoopNode(head1);
		Node loop2 = getLoopNode(head2);
		if (loop1 == null && loop2 == null) {
			return noLoop(head1, head2);
		}
		if (loop1 != null && loop2 != null) {
			return bothLoop(head1, loop1, head2, loop2);
		}
		return null;
	}

	// 找到链表第一个入环节点，如果无环，返回null
	public static Node getLoopNode(Node head) {
		if (head == null || head.next == null || head.next.next == null) {
			return null;
		}
		// n1 慢  n2 快
		Node slow = head.next; // n1 -> slow
		Node fast = head.next.next; // n2 -> fast
		while (slow != fast) {
			if (fast.next == null || fast.next.next == null) {
				return null;
			}
			fast = fast.next.next;
			slow = slow.next;
		}
		// slow fast  相遇
		fast = head; // n2 -> walk again from head
		while (slow != fast) {
			slow = slow.next;
			fast = fast.next;
		}
		return slow;
	}

	// 如果两个链表都无环，返回第一个相交节点，如果不想交，返回null
	public static Node noLoop(Node head1, Node head2) {
		if (head1 == null || head2 == null) {
			return null;
		}
		Node cur1 = head1;
		Node cur2 = head2;
		int n = 0;
		while (cur1.next != null) {
			n++;
			cur1 = cur1.next;
		}
		while (cur2.next != null) {
			n--;
			cur2 = cur2.next;
		}
		if (cur1 != cur2) {
			return null;
		}
		// n  :  链表1长度减去链表2长度的值
		cur1 = n > 0 ? head1 : head2; // 谁长，谁的头变成cur1
		cur2 = cur1 == head1 ? head2 : head1; // 谁短，谁的头变成cur2
		n = Math.abs(n);
		while (n != 0) {
			n--;
			cur1 = cur1.next;
		}
		while (cur1 != cur2) {
			cur1 = cur1.next;
			cur2 = cur2.next;
		}
		return cur1;
	}

	// 两个有环链表，返回第一个相交节点，如果不想交返回null
	public static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {
		Node cur1 = null;
		Node cur2 = null;
		if (loop1 == loop2) {
			cur1 = head1;
			cur2 = head2;
			int n = 0;
			while (cur1 != loop1) {
				n++;
				cur1 = cur1.next;
			}
			while (cur2 != loop2) {
				n--;
				cur2 = cur2.next;
			}
			cur1 = n > 0 ? head1 : head2;
			cur2 = cur1 == head1 ? head2 : head1;
			n = Math.abs(n);
			while (n != 0) {
				n--;
				cur1 = cur1.next;
			}
			while (cur1 != cur2) {
				cur1 = cur1.next;
				cur2 = cur2.next;
			}
			return cur1;
		} else {
			cur1 = loop1.next;
			while (cur1 != loop1) {
				if (cur1 == loop2) {
					return loop1;
				}
				cur1 = cur1.next;
			}
			return null;
		}
	}

	public static void main(String[] args) {
		// 1->2->3->4->5->6->7->null
		Node head1 = new Node(1);
		head1.next = new Node(2);
		head1.next.next = new Node(3);
		head1.next.next.next = new Node(4);
		head1.next.next.next.next = new Node(5);
		head1.next.next.next.next.next = new Node(6);
		head1.next.next.next.next.next.next = new Node(7);

		// 0->9->8->6->7->null
		Node head2 = new Node(0);
		head2.next = new Node(9);
		head2.next.next = new Node(8);
		head2.next.next.next = head1.next.next.next.next.next; // 8->6
		System.out.println(getIntersectNode(head1, head2).value);

		// 1->2->3->4->5->6->7->4...
		head1 = new Node(1);
		head1.next = new Node(2);
		head1.next.next = new Node(3);
		head1.next.next.next = new Node(4);
		head1.next.next.next.next = new Node(5);
		head1.next.next.next.next.next = new Node(6);
		head1.next.next.next.next.next.next = new Node(7);
		head1.next.next.next.next.next.next = head1.next.next.next; // 7->4

		// 0->9->8->2...
		head2 = new Node(0);
		head2.next = new Node(9);
		head2.next.next = new Node(8);
		head2.next.next.next = head1.next; // 8->2
		System.out.println(getIntersectNode(head1, head2).value);

		// 0->9->8->6->4->5->6..
		head2 = new Node(0);
		head2.next = new Node(9);
		head2.next.next = new Node(8);
		head2.next.next.next = head1.next.next.next.next.next; // 8->6
		System.out.println(getIntersectNode(head1, head2).value);

	}

}