algorithmbasic2020/src/leo/class10/FindFirstIntersectNode.java

package leo.class10;

class Node{
    int value;
    Node next;
    public Node(int v){
        this.value = v;
    }

}
/**
 * @author Leo
 * @ClassName FindFirstIntersectNode
 * @DATE 2020/12/4 2:26 下午
 * @Description
 * 给定两个可能有环也可能无环的单链表，头节点head1和head2。请实现一个函数，如果两个链表相交，请返回相交的 第一个节点。如果不相交，返回null
 * 【要求】
 * 如果两个链表长度之和为N，时间复杂度请达到O(N)，额外空间复杂度 请达到O(1)。
 */
public class FindFirstIntersectNode {

    public static Node getIntersectNode(Node head1, Node head2) {
        if (head1 == null || head2 == null) {
            return null;
        }
        // judgment node is there a ring
        Node loop1 = getLoop(head1);
        Node loop2 = getLoop(head2);

        //judgment different situations
        if (loop1 == null && loop2 == null) {
            //无环
            return noLoop(head1, head2);
        } else if (loop1 != null && loop2 != null) {
            //有环
            return bothLoop(head1, loop1, head2, loop2);
        }
        return null;
    }

    private static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {
        if (head1 == null || head2 == null) {
            return null;
        }
        Node cur1 = null;
        Node cur2 = null;
        if (loop1 == loop2) {
            cur1 = head1;
            cur2 = head2;
            int n = 0;
            while (cur1 != loop1) {
                cur1 = cur1.next;
                n++;
            }
            while (cur2 != loop2) {
                cur2 = cur2.next;
                n--;
            }
            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 Node getLoop(Node head) {
        if (head == null || head.next == null || head.next.next == null) {
            return null;
        }

        Node slow = head.next;
        Node fast = head.next.next;
        while (slow != fast) {
            if (fast.next == null || fast.next.next == null) {
                return null;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        fast = head;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }

    public static Node noLoop(Node head1, Node head2) {
        if (head1 == null || head2 == null) {
            return null;
        }
        int n = 0;
        Node cur1 = head1;
        Node cur2 = head2;
        while (cur1 != null) {
            n++;
            cur1 = cur1.next;
        }
        while (cur2 != null) {
            n--;
            cur2 = cur2.next;
        }
        if (cur1 != cur2) {
            return null;
        }
        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;

    }

}

class FindFirstIntersectNode1{
    public static Node getIntersectNode(Node head1, Node head2) {
        if (head1 == null || head2 == null) {
            return null;
        }
        Node loop1 = getLoop(head1);
        Node loop2 = getLoop(head2);
        if (loop1 != null && loop2 != null) {
            return bothLoop(head1, loop1, head2, loop2);
        } else if (loop1 == null && loop2 == null) {
            return noLoop(head1, head2);
        }
        return null;
    }

    public static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {
        Node cur1 = null;
        Node cur2 = null;
        if (loop1 == loop2) {
            int n = 0;
            cur1 = head1;
            cur2 = head2;
            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) {
                cur1 = cur1.next;
                n--;
            }
            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 Node noLoop(Node head1, Node head2) {
        if (head1 == null || head2 == null) {
            return null;
        }
        int n = 0;
        Node cur1 = head1;
        while (cur1 != null) {
            n++;
            cur1 = cur1.next;
        }
        Node cur2 = head2;
        while (cur2 != null) {
            n--;
            cur2 = cur2.next;
        }
        if (cur1 != cur2) {
            return null;
        }
        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;
    }

    public static Node getLoop(Node head) {
        if (head == null || head.next == null || head.next.next == null) {
            return null;
        }
        Node slow = head.next;
        Node fast = head.next.next;
        while (slow != fast) {
            if (fast.next == null || fast.next.next == null) {
                return null;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        fast = head;
        while (fast != slow) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }
}

class FindFirstIntersectNode2{

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

        }
        return null;

    }

    public static Node getLoop(Node head) {
        if (head == null || head.next == null || head.next.next == null) {
            return null;
        }
        Node slow = head.next;
        Node fast = head.next.next;
        while (slow != fast) {
            if (fast.next == null || fast.next.next == null) {
                return null;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        fast = head;
        while(fast!=slow){
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }

    private static Node noLoop(Node head1, Node head2) {
        int n = 0;
        Node cur1 = head1;
        Node cur2 = head2;
        while (cur1 != null) {
            cur1 = cur1.next;
            n++;
        }
        while (cur2 != null) {
            cur2 = cur2.next;
            n--;
        }
        if (cur1 != cur2) {
            return null;
        }
        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;
    }

    private static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {

        if (loop1 != loop2) {
            Node cur = loop1.next;
            while (cur != loop1) {
                if (cur == loop2) {
                    return loop1;
                }
                cur = cur.next;
            }
            return null;
        }else{
            Node cur1 = head1;
            Node cur2 = head2;
            int n = 0;
            while (cur1 != loop1) {
                n++;
                cur1 = cur1.next;
            }
            while (cur2 != loop2) {
                n--;
                cur2 = cur2.next;
            }
            if (cur1 != cur2) {
                return null;
            }
            cur1 = n > 0 ? head1 : head2;
            cur2 = cur1 == head1 ? head2 : head1;
            n = Math.abs(n);
            while (n != 0) {
                cur1 = cur1.next;
                n--;
            }
            while (cur1 != cur2) {
                cur1 = cur1.next;
                cur2 = cur2.next;
            }
            return cur1;
        }
    }
}

class FindFirstIntersectNode3{
    public static Node getIntersectNode(Node head1, Node head2) {
        if (head1 == null || head1.next == null || head1.next.next == null) {
            return null;
        }
        if (head2 == null || head2.next == null || head1.next.next == null) {
            return null;
        }
        Node loop1 = getLoop(head1);
        Node loop2 = getLoop(head2);
        if (loop1 == null && loop2 == null) {
            return noLoop(head1, head2);
        } else if (loop1 != null & loop2 != null) {
            return bothLoop(head1, loop1, head2, loop2);
        }
        return null;
    }

    public static Node getLoop(Node head) {
        if (head == null) {
            return null;
        }
        Node slow = head.next;
        Node fast = head.next.next;
        while (slow != fast) {
            if (fast.next == null || fast.next.next == null) {
                return null;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        fast = head;
        while (fast != slow) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }

    public static Node noLoop(Node head1, Node head2) {
        if (head1 == null || head1.next == null || head1.next.next == null) {
            return null;
        }
        if (head2 == null || head2.next == null || head2.next.next == null) {
            return null;
        }
        int n = 0;
        Node cur1 = head1;
        Node cur2 = head2;
        while (cur1 != null) {
            n++;
            cur1 = cur1.next;
        }
        while (cur2 != null) {
            n--;
            cur2 = cur2.next;
        }
        if (cur1 != cur2) {
            return null;
        }
        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;
    }


    public static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {
        Node cur1;
        Node cur2;
        if (loop1 == loop2) {
            cur1 = head1;
            cur2 = head2;
            int n = 0;
            while (cur1 != loop1) {
                cur1 = cur1.next;
                n++;
            }
            while (cur2 != loop2) {
                cur2 = cur2.next;
                n--;
            }
            if (cur1 != cur2) {
                return null;
            }
            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;
        }
    }
}

class FindFirstIntersectNode_Main{
    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(FindFirstIntersectNode3 .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(FindFirstIntersectNode3.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(FindFirstIntersectNode3.getIntersectNode(head1, head2).value);

    }
}