Four more Leetcode problems

Leetcode/19-remove-nth-node-from-end-of-list.cpp

@@ -0,0 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* removeNthFromEnd(ListNode* head, int n) {
+        ListNode* p1=head,*p2=head;
+        for(int i=0; i<n; i++) {
+            p2 = p2->next;
+        }
+        if (p2 == nullptr) return head->next;
+        while (p2->next != nullptr) {
+            p1 = p1->next;
+            p2 = p2->next;
+        }
+        auto tmp = p1-> next;
+        p1->next = tmp->next;
+        return head;
+    }
+};

Leetcode/287-find-the-duplicate-number.cpp

@@ -0,0 +1,27 @@
+class Solution {
+public:
+    /*
+        Solution explanation:
+        Short explanation: checkout turtle and rabbit algorithm on cycle detection
+        Complete explanation: TBD
+    */
+    int findDuplicate(vector<int>& nums) {
+        int slow=nums[0],fast=nums[0];
+
+        while (true) {
+            slow = nums[slow];
+            fast = nums[nums[fast]];
+            if (slow == fast) {
+                // do while is that you?
+                break;
+            }
+        }
+        slow = nums[0];
+        while (slow != fast) {
+            slow = nums[slow];
+            fast = nums[fast];
+        }
+
+        return slow;
+    }
+};

Leetcode/41-first-missing-positive.cpp

@@ -0,0 +1,24 @@
+class Solution {
+public:
+    int firstMissingPositive(vector<int>& nums) {
+        for(int index=0; index < nums.size(); index++) {
+            if (nums[index] > 0 && nums[index] < nums.size() + 1) {
+                int tmp = nums[nums[index]-1];
+                nums[nums[index] - 1] = nums[index];
+                nums[index] = tmp;
+            }
+        }
+        for(int index=0; index < nums.size(); index++) {
+            while (nums[index] > 0 && nums[index] < nums.size() + 1) {
+                if (nums[nums[index] - 1] == nums[index]) break;
+                int tmp = nums[nums[index]-1];
+                nums[nums[index] - 1] = nums[index];
+                nums[index] = tmp;
+            }
+        }
+        for(int index=0; index < nums.size(); index++) {
+            if (nums[index]!=index+1) return index+1;
+        }
+        return nums.size() + 1;
+    }
+};