Add merge intervals solution in merge_intervals.cpp

Implement merge intervals solution using sorting.

Author: ajithh404

merge_intervals.cpp

@@ -0,0 +1,18 @@
+// 56. Merge Intervals solution
+
+class Solution {
+ public:
+  vector<vector<int>> merge(vector<vector<int>>& intervals) {
+    vector<vector<int>> ans;
+
+    ranges::sort(intervals);
+
+    for (const vector<int>& interval : intervals)
+      if (ans.empty() || ans.back()[1] < interval[0])
+        ans.push_back(interval);
+      else
+        ans.back()[1] = max(ans.back()[1], interval[1]);
+
+    return ans;
+  }
+};