Create 4sum.cpp

Added code for the Leetcode Problem 4Sum.

leetcode/4Sum.cpp

@@ -0,0 +1,67 @@
+// 4 Sum - Leetcode
+// https://leetcode.com/problems/4sum/
+class Solution
+{
+public:
+    vector<vector<int>> fourSum(vector<int> &num, int target)
+    {
+        vector<vector<int>> res;
+
+        if (num.empty())
+            return res;
+        int n = num.size();
+        sort(num.begin(), num.end());
+
+        for (int i = 0; i < n; i++)
+        {
+
+            int target_3 = target - num[i];
+
+            for (int j = i + 1; j < n; j++)
+            {
+
+                int target_2 = target_3 - num[j];
+
+                int front = j + 1;
+                int back = n - 1;
+
+                while (front < back)
+                {
+
+                    int two_sum = num[front] + num[back];
+
+                    if (two_sum < target_2)
+                        front++;
+
+                    else if (two_sum > target_2)
+                        back--;
+
+                    else
+                    {
+
+                        vector<int> quadruplet(4, 0);
+                        quadruplet[0] = num[i];
+                        quadruplet[1] = num[j];
+                        quadruplet[2] = num[front];
+                        quadruplet[3] = num[back];
+                        res.push_back(quadruplet);
+
+                        while (front < back && num[front] == quadruplet[2])
+                            ++front;
+
+                        while (front < back && num[back] == quadruplet[3])
+                            --back;
+                    }
+                }
+
+                while (j + 1 < n && num[j + 1] == num[j])
+                    ++j;
+            }
+
+            while (i + 1 < n && num[i + 1] == num[i])
+                ++i;
+        }
+
+        return res;
+    }
+};