详情请参照B站：正月点灯笼视频讲解。

#include<cstdio>
#include<cmath>
struct node{
	int value;
	struct node* next;
};
int get_list_length(node* list){
	node* p = list;
	int cnt=0;
	while(p != NULL){
		p = p->next;
		cnt++;
	}
	return  cnt;
}

node* find_common_node(node* list1, node* list2){
	int length1 = get_list_length(list1);
	int length2 = get_list_length(list2);
	node* long_list = list1;
	node* short_list = list2;
	int offset = abs(length1 - length2);
	if(length2 > length1){
		long_list = list2;
		short_list = list1;
	}
	node* p1 = long_list, * p2 = short_list; 
	for(int i=0; i<offset; i++){
		p1 = p1->next;		
	}
	while(p1!=NULL){
		if(p1 == p2){
			return p1;
		}
		p1 = p1->next;
		p2 = p2->next;
	}
	return NULL;
}
int main(){
	node n1,n2,n3,n4,n5,n6,n7;
	n1.value = 1;
	n2.value = 2;
	n3.value = 3;
	n4.value = 4;
	n5.value = 5;
	n6.value = 6;
	n7.value = 7;
	
	
	n1.next = &n2;
	n2.next = &n3;
	n3.next = &n4;
	n4.next = &n5;
	n5.next = &n6;
	n6.next = NULL;
	n7.next = &n4;
	
	node* list1 = &n1;
	node* list2 = &n7;
	node* p = find_common_node(list1, list2);
	printf("%d\n",p->value);
}

Leetcode160

/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */
class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        ListNode* list1=headA,*list2=headB;
        int len1=0,len2=0;
        while(list1){
            len1++;
            list1=list1->next;
        }
        while(list2){
            len2++;
            list2=list2->next;
        }
        ListNode* long_list=headA,*short_list=headB;
        if(len2 > len1){
            long_list=headB;
            short_list=headA;
        }
        int offset=abs(len1 - len2);
        for(int i=0;i<offset;i++){
            long_list=long_list->next;
        }
        while(long_list&&short_list&&long_list!=short_list){
            long_list=long_list->next;
            short_list=short_list->next;
        }
        return long_list==short_list?long_list:0;
    }
};

其他风格

/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */
class Solution {
public:
    int getlength(ListNode* head){
        int r=0;
        while(head){
            ++r;
            head=head->next;
        }
       return r; 
    }
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        ListNode* list1=headA,*list2=headB;
        int lenA=getlength(headA),lenB=getlength(headB);
        if(lenA > lenB){
           for(int i=0;i<lenA-lenB;i++){
               headA=headA->next;
           }
        }else{
            for(int i=0;i<lenB-lenA;i++){
               headB=headB->next;
           }
        }
       
        while(headA&&headB&&headA!=headB){
            headA=headA->next;
            headB=headB->next;
        }
        return headA==headB? headA :0;
    }
};