Add solution to 'a lot' of problems

Easy/contains-duplicate-ii/solution.cpp

@@ -0,0 +1,27 @@
+#include <cmath>
+#include <unordered_map>
+#include <vector>
+
+class Solution {
+public:
+    bool containsDuplicate(std::vector<int> &nums, int k) {
+        std::unordered_map<int, std::vector<int>> unset;
+
+        int size = nums.size();
+        for (int i = 0; i < size; ++i) {
+            if (unset.find(nums[i]) != unset.end()) {
+                std::vector<int> indices = unset[nums[i]];
+
+                for (int j = 0; j < indices.size(); ++j) {
+                    if (std::abs(indices[j] - i) <= k) {
+                        return true;
+                    }
+                }
+            }
+
+            unset[nums[i]].push_back(i);
+        }
+
+        return false;
+    }
+};