C++ 代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *findFirstCommonNode(ListNode *headA, ListNode *headB) {
        if(!headA || !headB) return nullptr;

        // 求出两个链表的长度
        auto curA = headA, curB = headB;
        int lengthA = 0, lengthB = 0;
        while(curA) {curA = curA->next; lengthA++;}
        while(curB) {curB = curB->next; lengthB++;}

        // 较长链表先走差值步
        int n = abs(lengthA - lengthB);
        if(lengthA > lengthB) 
            while(n--) headA = headA->next;
        else 
            while(n--) headB = headB->next;

        // 两个链表一起走
        while(headA != headB)
        {
            headA = headA->next;
            headB = headB->next;
        }

        return headA;
    }
};