剑指offer之找到链表里面包含环的入口节点

1 问题

剑指offer之找到链表里面包含环的入口节点，比如

    //             node7<-node6 <-node5//              |              |//head->node1->node2->node3->node4

环的入口节点是node2

2 代码实现

#include <stdio.h>
#include <stdlib.h>#define true 1
#define false 0 typedef struct node
{int value;struct node *next;
} Node;/***得到环的第一个公共节点*/
Node *getCommonNode(Node *head)
{if (head == NULL){return NULL;}Node *first = NULL;Node *second = NULL;first = head;second = head;int isCircle = false;//判断是否有环while (second != NULL && (second->next) != NULL && (second->next->next != NULL)){first = first->next;second = second->next->next;if (first == second){isCircle = true;break;}}if (isCircle == false){printf("the list do not circle\n");return NULL;	}//判断环的大小,这个时候肯定是进到环里面去了int len = 0;first = first->next;++len;while (first != second){len++;first = first->next;}//求出入口节点Node *start = head;Node *end = head;while (len-- > 0){end = end->next;}while (start != end){start = start->next;end = end->next;}return start;
}int main()
{Node *head = NULL;Node *node1 = NULL;Node *node2 = NULL;Node *node3 = NULL;Node *node4 = NULL;Node *node5 = NULL;Node *node6 = NULL;Node *node7 = NULL;head = (Node *)malloc(sizeof(Node));node1 = (Node *)malloc(sizeof(Node));node2 = (Node *)malloc(sizeof(Node));node3 = (Node *)malloc(sizeof(Node));node4 = (Node *)malloc(sizeof(Node));node5 = (Node *)malloc(sizeof(Node));node6 = (Node *)malloc(sizeof(Node));node7 = (Node *)malloc(sizeof(Node));if (head == NULL || node1 == NULL || node2 == NULL || node3 == NULL|| node4 == NULL || node5 == NULL || node6 == NULL || node7 == NULL){printf("malloc fail\n");return false;}//             node7<-node6 <-node5//              |              |//head->node1->node2->node3->node4head->value = 0;head->next = node1;node1->value = 1;node1->next = node2;node2->value = 2;node2->next = node3;node3->value = 3;node3->next = node4;node4->value = 4;node4->next = node5;node5->value = 5;node5->next = node6;node6->value = 6;node6->next = node7;node7->value = 7;node7->next = node2;Node *result = getCommonNode(head);if (result != NULL){printf("the first common value is %d\n", result->value);}else{printf("list do not have circle\n");}return true;
}