金九銀十之兩個鏈表的相交的第一個節點

• 小知識，大挑戰！本文正在參與“程序員必備小知識”創作活動。

文字題解

找到第一個節點要分有環跟無環兩種情況

代碼實現

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

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

} private 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; }

/* * 兩個無環鏈表,返回第一個相交節點,如果不相交就返回null * @param head1 * @param head2 * @return / private 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; } 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; }

/* * 兩個有環鏈表,返回第一個相交節點,如果不相交就返回null * * @param head1 * @param loop1 * @param head2 * @param loop2 * @return / private 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; } }

/* * @param head 找到入環節點 * @return / public static Node getLoopNode(Node head) { Node n1 = head.next; // n1 -> slow Node n2 = head.next.next; // n2 -> fast while (n1 != n2) { if (n2.next == null || n2.next.next == null) { return null; } n2 = n2.next.next; n1 = n1.next; } n2 = head; // n2 -> walk again from head while (n1 != n2) { n1 = n1.next; n2 = n2.next; } return n1; } ```