Linked list cycle

Signed-off-by: Leo Ma <[email protected]>

Author: begeekmyfriend

141_linked_list_cycle/Makefile

@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test list_cycle.c

141_linked_list_cycle/list_cycle.c

@@ -0,0 +1,47 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+struct ListNode {
+    int val;
+    struct ListNode *next;
+};
+
+static bool hasCycle(struct ListNode *head)
+{
+    if (head == NULL || head->next == NULL) {
+        return false;
+    }
+
+    bool first = true;
+    struct ListNode *p0, *p1;
+    for (p0 = head, p1 = head; p1 != NULL && p1->next != NULL; p0 = p0->next, p1 = p1->next->next) {
+        if (p0 == p1 && !first) {
+            return true;
+        }
+        first = false;
+    }
+
+    return false;
+}
+
+int main(int argc, char **argv)
+{
+    int i, count = argc - 1;
+    struct ListNode *head = NULL, *p, *prev;
+    for (i = 0; i < count; i++) {
+        p = malloc(sizeof(*p));
+        p->val = atoi(argv[i + 1]);
+        p->next = NULL;
+        if (head == NULL) {
+            head = p;
+        } else {
+            prev->next = p;
+        }
+        prev = p;
+    }
+    p->next = head;
+
+    printf("%s\n", hasCycle(head) ? "true" : "false");
+    return 0;
+}

142_linked_list_cycle_ii/Makefile

@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test list_cycle.c

142_linked_list_cycle_ii/list_cycle.c

@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+struct ListNode {
+    int val;
+    struct ListNode *next;
+};
+
+static struct ListNode *detectCycle(struct ListNode *head)
+{
+    if (head == NULL || head->next == NULL) {
+        return false;
+    }
+
+    bool first = true;
+    struct ListNode *p0, *p1;
+    for (p0 = head, p1 = head; p1 != NULL && p1->next != NULL; p0 = p0->next, p1 = p1->next->next) {
+        if (p0 == p1 && !first) {
+            p0 = head;
+            while (p0 != p1) {
+                p0 = p0->next;
+                p1 = p1->next;
+            }
+            return p0;
+        }
+        first = false;
+    }
+
+    return NULL;
+}
+
+int main(int argc, char **argv)
+{
+    int i, count = argc - 1;
+    struct ListNode *head = NULL, *p, *prev;
+    for (i = 0; i < count; i++) {
+        p = malloc(sizeof(*p));
+        p->val = atoi(argv[i + 1]);
+        p->next = NULL;
+        if (head == NULL) {
+            head = p;
+        } else {
+            prev->next = p;
+        }
+        prev = p;
+    }
+    p->next = head;
+
+    p = detectCycle(head);
+    if (p != NULL) {
+        printf("%d\n", p->val);
+    }
+    return 0;
+}