如何计算3到7个字符的排列

def permute(chars, start, end):
    if start == end:
        print(''.join(chars))
    else:
        for i in range(start, end+1):
            chars[start], chars[i] = chars[i], chars[start]
            permute(chars, start+1, end)
            chars[start], chars[i] = chars[i], chars[start]   # 恢复原序列，进行下一次交换

def calculate_permutations(n):
    if n < 3 or n > 7:
        print('字符个数范围必须在3到7之间')
        return
    chars = []
    for i in range(n):
        chars.append(chr(ord('a')+i))   # 假设字符从'a'开始递增
    permute(chars, 0, n-1)

calculate_permutations(3)
calculate_permutations(4)
calculate_permutations(5)
calculate_permutations(6)
calculate_permutations(7)