题解 | #链表中环的入口结点#

/**
 * struct ListNode {
 *  int val;
 *  struct ListNode *next;
 * };
 */
/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 *
 * @param pHead ListNode类
 * @return ListNode类
 */
struct ListNode* EntryNodeOfLoop(struct ListNode* pHead) {
    // write code here
    if (pHead == NULL)
        return pHead;
    struct ListNode* fast = pHead;
    struct ListNode* slow = pHead;
    while (fast != NULL && fast->next != NULL) {
        fast = fast->next->next;
        slow = slow->next;
        if (fast == slow) {
            struct ListNode* index1 = pHead;
            struct ListNode* index2 = slow;
            while (index1 != index2) {
                index1 = index1->next;
                index2 = index2->next;
            }
            return index1;
        }
    }
    return NULL;
}