Add determine if the linked list has a cycle in it.

Author: chachaxw

leetcode/linked_list/linked_list_cycle.cpp

@@ -0,0 +1,49 @@
+/**
+ * @Author: Chacha 
+ * @Date: 2019-01-14 15:24:40 
+ * @Last Modified by: Chacha
+ * @Last Modified time: 2019-01-14 15:37:33
+ */
+
+
+#include<iostream>
+#include<string>
+using namespace std;
+
+/** 
+ * Definition for singly-linked list(单向链表)
+ * Source: https://zh.wikipedia.org/wiki/链表
+ */
+struct ListNode {
+    int val;
+    ListNode *next;
+    ListNode(int x) : val(x), next(NULL) {}
+};
+
+class Solution {
+public:
+    /**
+     * Given a linked list, determine if it has a cycle in it.
+     * Example: Given -21->10->4->5, tail connects to node index 1, return true
+     * Source: https://leetcode.com/problems/linked-list-cycle/
+     */ 
+    bool hasCycle(ListNode* head) {
+        if (head == NULL || head->next == NULL) return false;
+
+        ListNode* slow = head;
+        ListNode* fast = head->next;
+
+        while (fast != NULL && fast->next != NULL) {
+            fast = fast->next->next;
+            slow = slow->next;
+
+            if (slow == fast) return true;
+        }
+
+        return false;
+    }
+};
+
+int main() {
+    return 0;
+}