160. Intersection of Two Linked Lists【easy】

160. Intersection of Two Linked Lists【easy】

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.

解法一：

class Solution {
public:
    /**
     * @param headA: the first list
     * @param headB: the second list
     * @return: a ListNode
     */
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        // write your code here
        if(headA == NULL || headB == NULL)
            return NULL;
        ListNode* iter1 = headA;
        ListNode* iter2 = headB;
        int len1 = 1;
        while(iter1->next != NULL)
        {
            iter1 = iter1->next;
            len1 ++;
        }
        int len2 = 1;
        while(iter2->next != NULL)
        {
            iter2 = iter2->next;
            len2 ++;
        }
        if(iter1 != iter2)
            return NULL;
        if(len1 > len2)
        {
            for(int i = 0; i < len1-len2; i ++)
                headA = headA->next;
        }
        else if(len2 > len1)
        {
            for(int i = 0; i < len2-len1; i ++)
                headB = headB->next;
        }
        while(headA != headB)
        {
            headA = headA->next;
            headB = headB->next;
        }
        return headA;
    }
};

先算长度，然后长的先走差值步，然后同时走

解法二：

public class Solution {
    /**
     * @param headA: the first list
     * @param headB: the second list
     * @return: a ListNode 
     */
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null) {
            return null;
        }
        
        // get the tail of list A.
        ListNode node = headA;
        while (node.next != null) {
            node = node.next;
        }
        node.next = headB;
        ListNode result = listCycleII(headA);
        node.next = null;
        return result;
    }
    
    private ListNode listCycleII(ListNode head) {
        ListNode slow = head, fast = head.next;
        
        while (slow != fast) {
            if (fast == null || fast.next == null) {
                return null;
            }
            
            slow = slow.next;
            fast = fast.next.next;
        }
        
        slow = head;
        fast = fast.next;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        
        return slow;
    }
}

先弄成环，转换为找环的入口问题，找到之后再断开环

 

找环的问题解法可以参见（142. Linked List Cycle II【easy】）