Add Web Crawler solution

solutions/system_design/web_crawler/web_crawler_mapreduce.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+
+from mrjob.job import MRJob
+
+
+class RemoveDuplicateUrls(MRJob):
+
+    def mapper(self, _, line):
+        yield line, 1
+
+    def reducer(self, key, values):
+        total = sum(values)
+        if total == 1:
+            yield key, total
+
+    def steps(self):
+        """Run the map and reduce steps."""
+        return [
+            self.mr(mapper=self.mapper,
+                    reducer=self.reducer)
+        ]
+
+
+if __name__ == '__main__':
+    RemoveDuplicateUrls.run()