题解 | #两个链表的第一个公共结点#

主要思路：寻找两个链表之间长度的差值。让长的链表移动一定的差值距离，使得此时两个指针遍历的长度一致，如果在遍历的过程中出现两个指针相等，说明存在相同节点。

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
  public:
    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        int len1 = 0, len2 = 0, sub = 0;
        ListNode* cur1 = pHead1, *cur2 = pHead2;

        while (cur1 != NULL) {
            len1++;
            cur1 = cur1->next;
        }
        while (cur2 != NULL) {
            len2++;
            cur2 = cur2->next;
        }
        cur1 = pHead1;
        cur2 = pHead2;
        sub = abs(len1 - len2);
        if (len1 > len2) {
            while (sub--) {
                cur1 = cur1->next;
            }
        } else {
            while (sub--) {
                cur2 = cur2->next;
            }
        }

        while (cur1 != NULL && cur2 != NULL) {
            if (cur1 == cur2) {
                return cur1;
            }

            cur1 = cur1->next;
            cur2 = cur2->next;
        }
        return NULL;
    }
};