Some .c files contain cpp syntax.

[H-Shen]

Found by:

find . -type f -name \*.c | xargs grep -E "using namespace std"

On:

./Hackerrank/Mathematics/Connecting_Towns.c:using namespace std;
./PTA_Basic_Programming_Problemset/6-13.c:using namespace std;
./PTA_Basic_Programming_Problemset/7-3.c:using namespace std;
./PTA_Basic_Programming_Problemset/7-2.c:using namespace std;
./PTA_Basic_Programming_Problemset/7-1.c:using namespace std;

[H-Shen]

Fixed.