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
    struct ListNode *f = NULL;
    if(pHead == NULL) return f;
    f = pHead;
    struct ListNode* s = pHead;
    bool haveCycle = false;
    while(f->next != NULL && f->next->next != NULL) {
        f = f->next->next;
        s = s->next;
        if(f == s) {
            haveCycle = true;
            break;
        }
    }
    if(haveCycle) {
        f = pHead;
        while(f != s) {
            f = f->next;
            s = s->next;
        }
    } else {
        f = NULL;
    }
    return f;
}