剑指offer (37) 两个链表的第一个公共结点

ListNode* FirstCommonNode(ListNode* list1, ListNode* list2)
{
    if (list1 == NULL || list2 == NULL)  return NULL;
    ListNode* cur1 = list1;
    ListNode* cur2 = list2;
    int list1Len = 0;
    int list2Len = 0;
    while (cur1++ != NULL) { ++list1Len; }
    while (cur2++ != NULL) { ++list2Len; }
    int maxLen = std::max(list1Len, list2Len);
    ListNode* longerList = NULL;
    ListNode* shorterList = NULL;
    int gap = 0;
    if (maxLen == list1Len) {
        longerList  = list1;
        shorterList = list2;
        gap = maxLen - list2Len;
    } else {
        longerList  = list2;
        shorterList = list1;
        gap = maxLen - list1Len;
    }

    int firstStep = 0;
    while (firstStep != gap) {
        longerList = longerList->next;
        ++firstStep;
    }

    while (longerList != NULL && shorterList != NULL) {
        if (longerList == shorterList) {
            return longerList;
        } else {
            longerList  = longerList->next;
            shorterList = shorterList->next;
        }
    }
    return NULL;
}