## Find the next small number

There is a very interesting problem.

Write a function that takes a positive integer and returns the next smaller positive integer containing the same digits.

Next smaller number with the same digits

```
def next_smaller(n):
s = list(str(n))
# find the pivot
i = j = len(s) - 1
while i > 0 and s[i - 1] <= s[i]:
i -= 1
# if pivot is -1 means it is descending e.g. 1,2,3,4,5
# which means it is already the smallest
if i <= 0: return -1
# find the substitute of pivot
while s[j] >= s[i - 1]:
j -= 1
# replace pivot with substitute
s[i - 1], s[j] = s[j], s[i - 1]
s[i:] = reversed(s[i:])
# there is no number start with zero
if s[0] == '0': return -1
return int(''.join(s))
```