python回溯法实现数组全排列输出实例分析

from sys import stdout
#code from http://www.bitsCN.com/
def perm(li, start, end):
 if(start == end):
 for elem in li:
 stdout.write(elem)
 print ''
 else:
 for i in range(start, end):
 li[start], li[i] = li[i], li[start]
 perm(li, start+1, end)
 li[i], li[start] = li[start], li[i]
if __name__ == '__main__':
 li = ['a','b','c','d']
 perm(li, 0, len(li))