基本算法4

全排列：
s=['a','b','c']
from copy import copy
ret=[]
len_input=len(s)
def changes(s,start,end):
    if end <1:
        ret.append( copy(s))
        return
    for i in range(end):
        s[start],s[start+i]=s[start+i],s[start]
        changes(s,start+1,end-1)
        s[start],s[start+i]=s[start+i],s[start]