Create counting-elements.cpp

Author: kamyu104

C++/counting-elements.cpp

@@ -0,0 +1,34 @@
+// Time:  O(n)
+// Space: O(n)
+
+class Solution {
+public:
+    int countElements(vector<int>& arr) {
+        unordered_set<int> lookup(cbegin(arr), cend(arr));
+        return count_if(cbegin(arr), cend(arr),
+                        [&lookup](const auto& x) {
+                            return lookup.count(x + 1);
+                        });
+    }
+};
+
+// Time:  O(nlogn)
+// Space: O(1)
+class Solution2 {
+public:
+    int countElements(vector<int>& arr) {
+        sort(begin(arr), end(arr));
+        int result = 0, l = 1;
+        for (int i = 0; i + 1 < arr.size(); ++i) {
+            if (arr[i] == arr[i + 1]) {
+                ++l;
+                continue;
+            }
+            if (arr[i] + 1 == arr[i + 1]) {
+                result += l;
+            }
+            l = 1;
+        }
+        return result;
+    }
+};