diff --git a/src/control/double.cc b/src/control/double.cc
index bb9dba1..565277d 100644
--- a/src/control/double.cc
+++ b/src/control/double.cc
@@ -1,3 +1,4 @@
+#include <unordered_map>
 #include "double.h"
 
 /**
@@ -5,33 +6,20 @@
  *
  * @param n
  * @return the sum of all values squared from 0 to n
- */
+ */ 
 long
 DoubleForLoop::SumSquare(int n) {
   long sum = 0;
   for (int i = 0; i < n; i += 1) {
-    for (int j = 0; j < n; j += 1) {
-      if (i == j) {
-        sum = sum + (long) (i * j);
-      }
-    }
+    sum += (long) (i * i);
   }
   return sum;
 }
 
-/**
- * @brief Sums all triangle numbers from T(1) to T(n)
- *
- * @param n
- * @return the sum of all triangle numbers from T(1) to T(n)
- */
-long
-DoubleForLoop::SumTriangle(int n) {
+long DoubleForLoop::SumTriangle(int n) {
   long sum = 0;
-  for (int i = 0; i < n + 1; i += 1) {
-    for (int j = 0; j < i; j += 1) {
-      sum = sum + (long) j;
-    }
+  for (int i = 0; i <= n; i++) {
+    sum += (i * (i - 1)) / 2;
   }
   return sum;
 }
@@ -44,22 +32,20 @@ DoubleForLoop::SumTriangle(int n) {
  *
  * @param v
  * @return the number of pairs in an vay
- */
+ */ 
 int
 DoubleForLoop::CountPairs(std::vector<int> v) {
-  int count = 0;
+  std::unordered_map<int, int> counts;
   for (int i = 0; i < (int) v.size(); i += 1) {
-    int nDuplicates = 0;
-    for (int j = 0; j < (int) v.size(); j += 1) {
-      if (v[i] == v[j]) {
-        nDuplicates += 1;
-      }
-    }
-    if (nDuplicates == 2) {
-      count += 1;
+    counts[v[i]]++;
+  }
+  int nPairs = 0;
+  for (auto it = counts.begin(); it != counts.end(); ++it) {
+    if (it->second == 2) {
+      nPairs++;
     }
   }
-  return count / 2;
+  return nPairs;
 }
 
 /**
@@ -98,4 +84,4 @@ DoubleForLoop::SumMatrix(std::vector<std::vector<int>> matrix) {
     }
   }
   return sum;
-}
+}
\ No newline at end of file
diff --git a/src/control/single.cc b/src/control/single.cc
index 06ad4a9..4b4fc1d 100644
--- a/src/control/single.cc
+++ b/src/control/single.cc
@@ -5,18 +5,11 @@
  *
  * @param n the upper bound (non-inclusive)
  * @return the sum of all integer values from 0 to n
- */
+ */ 
 int
 SingleForLoop::SumRange(int n) {
-  int array[n];
-  int sum = 0;
-  for (int i = 0; i < n; i += 1) {
-    array[i] = i;
-  }
-  for (int i = 0; i < n; i += 1) {
-    sum += array[i];
-  }
-  return sum;
+    // Use the formula for the sum of arithmetic sequence
+    return (n * (n - 1)) / 2;
 }
 
 /**
@@ -47,18 +40,12 @@ SingleForLoop::MaxVector(std::vector<int> &arr) {
  * @param n the upper bound (non-inclusive)
  * @param m the modulus
  * @return the sum of all values from 0 to n that are divisible by m
- */
+ */ 
 int
 SingleForLoop::SumModulus(int n, int m) {
-  int array[n];
   int sum = 0;
-  for (int i = 0; i < n; i += 1) {
-    array[i] = i;
-  }
-  for (int i = 0; i < n; i += 1) {
-    if (array[i] % m == 0) {
-      sum += array[i];
-    }
+  for (int i = 0; i < n; i += m) {
+    sum += i;
   }
   return sum;
-}
+}
\ No newline at end of file
diff --git a/src/datastructures/vector.cc b/src/datastructures/vector.cc
index 9f2c72d..44a72dc 100644
--- a/src/datastructures/vector.cc
+++ b/src/datastructures/vector.cc
@@ -1,3 +1,4 @@
+#include <algorithm>
 #include "vector.h"
 
 #include <iostream>
@@ -77,21 +78,11 @@ OpsVector::SearchVector(std::vector<int> &v, int n) {
  *
  * @param v Vector to sort.
  * @return The sorted vector.
- */
+ */ 
 std::vector<int>
 OpsVector::SortVector(std::vector<int> &v) {
-  std::vector<int> ret(v);
-
-  for (int i = 0; i < (int) ret.size(); i += 1) {
-    for (int j = 0; j < (int) ret.size() - 1; j += 1) {
-      if (ret[j] > ret[j + 1]) {
-        int temp = ret[j];
-        ret[j] = ret[j + 1];
-        ret[j + 1] = temp;
-      }
-    }
-  }
-  return ret;
+  std::sort(v.begin(), v.end());
+  return v;
 }
 
 /**
@@ -149,4 +140,4 @@ OpsVector::MergeVectors(std::vector<int> &v1, std::vector<int> &v2) {
   }
 
   return ret;
-}
+}
\ No newline at end of file