两个链接列表中的第一个公共元素

给定两个链表，找到给定链表之间的第一个公共元素，即，我们需要找到第一个列表的第一个节点，该第一个节点也存在于第二个列表中。
例子：

Input :  
   List1: 10->15->4->20
   Lsit2:  8->4->2->10
Output : 10

Input : 
   List1: 1->2->3->4
   Lsit2:  5->6->3->8
Output : 3

我们遍历第一个列表，对于每个节点，我们在第二个列表中搜索它。一旦在第二个列表中找到一个元素，我们就将其返回。

C++

// C++ program to find first common element in
// two unsorted linked list
#include 
using namespace std;
  
/* Link list node */
struct Node {
    int data;
    struct Node* next;
};
  
/* A utility function to insert a node at the 
   beginning of a linked list*/
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node = 
          (struct Node*)malloc(sizeof(struct Node));
    new_node->data = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}
  
/* Returns the first repeating element in linked list*/
int firstCommon(struct Node* head1, struct Node* head2)
{
    // Traverse through every node of first list
    for (; head1 != NULL; head1=head1->next)
  
       // If current node is present in second list
       for (Node *p = head2; p != NULL; p = p->next)
            if (p->data == head1->data)
                return head1->data;
  
    // If no common node
    return 0;
}
  
// Driver code
int main()
{
    struct Node* head1 = NULL;
    push(&head1, 20);
    push(&head1, 5);
    push(&head1, 15);
    push(&head1, 10);
  
    struct Node* head2 = NULL;
    push(&head2, 10);
    push(&head2, 2);
    push(&head2, 15);
    push(&head2, 8);
  
    cout << firstCommon(head1, head2);
    return 0;
}

Java

// Java program to find first common element 
// in two unsorted linked list
import java.util.*;
  
class GFG 
{
  
/* Link list node */
static class Node 
{
    int data;
    Node next;
};
  
/* A utility function to insert a node at the 
beginning of a linked list*/
static Node push(Node head_ref, int new_data)
{
    Node new_node = new Node();
    new_node.data = new_data;
    new_node.next = head_ref;
    head_ref = new_node;
    return head_ref;
}
  
/* Returns the first repeating element
   in linked list*/
static int firstCommon(Node head1, Node head2)
{
    // Traverse through every node of first list
    for (; head1 != null; head1 = head1.next)
  
    // If current node is present in second list
    for (Node p = head2; p != null; p = p.next)
            if (p.data == head1.data)
                return head1.data;
  
    // If no common node
    return 0;
}
  
// Driver code
public static void main(String[] args)
{
    Node head1 = null;
    head1 = push(head1, 20);
    head1 = push(head1, 5);
    head1 = push(head1, 15);
    head1 = push(head1, 10);
  
    Node head2 = null;
    head2 = push(head2, 10);
    head2 = push(head2, 2);
    head2 = push(head2, 15);
    head2 = push(head2, 8);
  
    System.out.println(firstCommon(head1, head2));
}
} 
  
// This code is contributed by 29AjayKumar

Python3

# Python3 program to find first common element 
# in two unsorted linked list
import math
  
# Link list node 
class Node: 
    def __init__(self, data): 
        self.data = data 
        self.next = None
  
# A utility function to insert a node at the 
#beginning of a linked list
def push(head_ref, new_data):
    new_node = Node(new_data)
    new_node.data = new_data
    new_node.next = head_ref
    head_ref = new_node
    return head_ref
  
# Returns the first repeating element
# in linked list*/
def firstCommon(head1, head2):
      
    # Traverse through every node of first list
    while(head1 != None):
        p = head2
  
    # If current node is present in second list
        while(p != None):
            if (p.data == head1.data):
                return head1.data
            p = p.next
        head1 = head1.next
          
    # If no common node
    return 0
  
# Driver code
if __name__=='__main__': 
    head1 = None
    head1 = push(head1, 20)
    head1 = push(head1, 5)
    head1 = push(head1, 15)
    head1 = push(head1, 10)
  
    head2 = None
    head2 = push(head2, 10)
    head2 = push(head2, 2)
    head2 = push(head2, 15)
    head2 = push(head2, 8)
  
    print(firstCommon(head1, head2))
  
# This code is contributed by Srathore

C#

// C# program to find first common element 
// in two unsorted linked list
using System;
using System.Collections.Generic;
      
class GFG 
{
  
/* Link list node */
class Node 
{
    public int data;
    public Node next;
};
  
/* A utility function to insert a node at the 
beginning of a linked list*/
static Node push(Node head_ref, int new_data)
{
    Node new_node = new Node();
    new_node.data = new_data;
    new_node.next = head_ref;
    head_ref = new_node;
    return head_ref;
}
  
/* Returns the first repeating element
in linked list*/
static int firstCommon(Node head1, Node head2)
{
    // Traverse through every node of first list
    for (; head1 != null; head1 = head1.next)
  
    // If current node is present in second list
    for (Node p = head2; p != null; p = p.next)
            if (p.data == head1.data)
                return head1.data;
  
    // If no common node
    return 0;
}
  
// Driver code
public static void Main(String[] args)
{
    Node head1 = null;
    head1 = push(head1, 20);
    head1 = push(head1, 5);
    head1 = push(head1, 15);
    head1 = push(head1, 10);
  
    Node head2 = null;
    head2 = push(head2, 10);
    head2 = push(head2, 2);
    head2 = push(head2, 15);
    head2 = push(head2, 8);
  
    Console.WriteLine(firstCommon(head1, head2));
}
} 
  
// This code is contributed by PrinciRaj1992

输出：

10