1?問題

劍指offer之找到鏈表里面包含環的入口節點，比如

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

環的入口節點是node2

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2?代碼實現

#include <stdio.h> #include <stdlib.h>#define true 1 #define false 0 typedef struct node {int value;struct node *next; } Node;/***得到環的第一個公共節點*/ Node *getCommonNode(Node *head) {if (head == NULL){return NULL;}Node *first = NULL;Node *second = NULL;first = head;second = head;int isCircle = false;//判斷是否有環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; }//判斷環的大小,這個時候肯定是進到環里面去了int len = 0;first = first->next;++len;while (first != second){len++;first = first->next;}//求出入口節點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; }

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3?運行結果

the first common value is 2

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總結

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