# Program to find kth smallest n length lexicographically smallest string in python

Suppose we have a number n and another value k. Now let us consider a string containing only "0", "1", and "2"s where no character is repeated in succession. We have to select such strings of length n and find the kth lexicographically smallest string. If there is no kth string, return empty string.

So, if the input is like n = 4 k = 2, then the output will be "0120".

To solve this, we will follow these steps:

• define a method solve() this will take s, k and last
• if s is same as 0, then
• return blank string
• for each character c in "012", do
• if c is same as last, then
• go for next iteration
• if k < 2^(s-1), then
• return c + solve(s - 1, k, c)
• k := k - 2^(s-1)
• return blank string
• From the main method call solve(n, k, Null)

Let us see the following implementation to get better understanding:

## Example Code

Live Demo

class Solution:
def solve(self, s, k, last=None):
if s == 0:
return ""
for c in "012":
if c == last:
continue
if k < 2 ** (s - 1):
return c + self.solve(s - 1, k, c)
k -= 2 ** (s - 1)
return ""

ob = Solution()
n = 4
k = 2
print(ob.solve(n, k))

4, 2

## Output

0120

Updated on: 25-Nov-2020

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