add dynamic processing assemble problem
add some comment for DP_assemble.c
modified the way to print shortest path
add matrix multipy programe DP_matrix.c
modified some comments of DP_matrix.c
add LCS programe DP_lcs.c
add the programe 15.4-5.c
modifed file 15.4-5.c
add file 15.4-6.c
add file 15.5.c
add the exercises of chapter 15
Merge branch 'DP'
Merge branch 'master' of github.com:zhanjl/Algorithm_learning