[Swift]LeetCode160. 相交链表 | Intersection of Two Linked Lists

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Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

---

编写一个程序，找到两个单链表相交的起始节点。

例如，下面的两个链表：

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

在节点 c1 开始相交。

注意：

• 如果两个链表没有交点，返回 nil.
• 在返回结果后，两个链表仍须保持原有的结构。
• 可假定整个链表结构中没有循环。
• 程序尽量满足 O(n) 时间复杂度，且仅用 O(1) 内存。

---

class Solution {
    func getIntersectionNode(_ headA: ListNode?, _ headB: ListNode? ) -> ListNode? {
        if headA == nil || headB == nil {
            return nil
        }
        
        var a = headA
        var b = headB
        while (a !== b) {
            if a != nil {
                a = a?.next
            } else {
                a = headB
            }
            
            if b != nil {
                b = b?.next
            } else {
                b = headA
            }
        }
        return a
    }
    
    public class ListNode: Equatable {
        
        public var val: Int
        public var next: ListNode?
        
        public init(_ val: Int) {
            self.val = val
            self.next = nil
        }
        
        public static func ==(lhs: Solution.ListNode, rhs: Solution.ListNode) -> Bool {
            return lhs.val == rhs.val && lhs.next == rhs.next
        }  
    }
}