找到两链表的第一个公共节点

【题目描述】

    输入两个链表，找出它们的第一个公共结点。

        For example, the following two linked lists:

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

begin to intersect at node c1.

 

【解题思路】

    如果两个链表在某一个节点处汇合，那么其后的部分将成为二者的公共部分。

 

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode *pHead1, ListNode *pHead2) {
        if(pHead1==NULL||pHead2==NULL)
            return NULL;
        int listLen1=0;
        int listLen2=0;
        ListNode * pNode1=pHead1;
        ListNode * pNode2=pHead2;
        while(pNode1->next!=NULL)
        {
            pNode1=pNode1->next;
            ++listLen1;
        }
        while(pNode2->next!=NULL)
        {
            pNode2=pNode2->next;
            ++listLen2;
        }

        if(pNode1!=pNode2)
            return NULL;
        else
        {
        int count=listLen1>listLen2?listLen1-listLen2:listLen2-listLen1;
        if(listLen1>listLen2)
        {
            pNode1=pHead1;
            pNode2=pHead2;
        }
        else
        {
            pNode1=pHead2;
            pNode2=pHead1;
        }

        while(count)
        {
            pNode1=pNode1->next;
            --count;
        }

        while(pNode1->next!=NULL&&pNode1!=pNode2)
        {
            pNode1=pNode1->next;
            pNode2=pNode2->next;
        }
        return pNode1;
        }    
    }
};

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.

【代码实现】