introduce overloaded iter_swap into std namespace which is bad, but it's ok for testing
bad code style: I need pass a parameter to program, so that I needn't comment the codes for output format
first time to add radixsort.cc
fix builder
1 added slice.h for testing string
2 rename some functions, make compiler happy
2 move random_string to random.h, and remove redundant codes
test on slice
first time add demo.c from "fast algorithms for sorting and searching strings" by Jon Bentely and Robert Sedqewick
bad!!!