add: duplicate number

Author: Ashutosh00710

Diff for: array/DuplicateNumber.cpp

@@ -0,0 +1,53 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+class Solution {
+public:
+    // using hash table to store the number of times of each number
+    // Time complexity: O(n)
+    // Space complexity: O(n)
+    int findDuplicateUsingSet(vector<int>& nums) {
+      set<int> s;
+      int res = 0;
+      for (int i = 0; i < nums.size(); i++) {
+        if (s.find(nums[i]) != s.end()) {
+          res = nums[i];
+          break;
+        } else {
+          s.insert(nums[i]);
+        }
+      }
+      return res;
+    }
+
+    // Floyd's Algorithm
+    // Time complexity: O(n)
+    // Space complexity: O(1)
+    int findDuplicate(vector<int>& nums) {
+      int slow = nums[0], fast = nums[0];
+      do {
+        slow = nums[slow];
+        fast = nums[nums[fast]];
+      } while (slow != fast);
+      fast = nums[0]; // reset the fast pointer to the beginning of the array
+      while(slow != fast) {
+        slow = nums[slow];
+        fast = nums[fast];
+      }
+      return slow;
+    }
+};
+
+int main() {
+    Solution sol;
+    vector<int> test1 = { 1,3,4,2,2 };
+    vector<int> test2 = { 3,1,3,4,2 };
+    vector<int> test3 = { 1,1 };
+    vector<int> test4 = { 1,1,2 };
+    cout<<sol.findDuplicate(test1)<<endl;
+    cout<<sol.findDuplicate(test2)<<endl;
+    cout<<sol.findDuplicate(test3)<<endl;
+    cout<<sol.findDuplicate(test4)<<endl;
+    return 0;
+}