adding lab 7

labs/07_list_implementation/README.md

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+# Lab 7 — List Implementation
+
+## Checkpoint 1
+*estimate: 20-30 minutes*
+
+The implementation of the dslist class is incomplete. In particular, the class is missing the destroy_list
+private member function that is used by the destructor and the clear member function. The provided test
+case in [checkpoint1.cpp](checkpoint1.cpp) works “fine”, so what’s the problem?
+
+Before we fix the problem, let’s use Dr. Memory and/or Valgrind to look at the details more carefully.
+You should use the memory debugging tools both on your local machine and by submitting the files to
+the homework server. Study the memory debugger output carefully. The output should match your
+understanding of the problems caused by the missing destroy_list implementation. Ask a TA if you
+have any questions.
+
+Now write and debug the destroy_list function and then re-run the memory debugger (both locally and on
+the submission server) to show that the memory problems have been fixed. Also finish the implementation
+of the push_front, pop_front, and pop_back functions.
+
+**To complete this checkpoint**, show a TA the implementation and memory debugger output before and
+after writing destroy_list.
+
+## Checkpoint 3: 
+*estimate: TBD*
+
+Merge Sort an Array.