Find Smallest Letter Greater Than Target in Python

Suppose we have a list of sorted characters’ letters. This is containing only lowercase letters, now we have a target letter t, we have to find the smallest element in the list that is larger than the given target.

And letters also wrap around. So, if the target is t = 'z' and letters = ['a', 'b'], the answer is 'a'.

So, if the input is like ["c", "f", "j"], t = 'a', then the output will be 'c'.

To solve this, we will follow these steps −

• l := 0
• r := size of letters - 1
• while l <= r, do
  • mid :=(l + r) / 2 as integer
  • if letters[mid] > target, then
    • r := mid -1
  • otherwise,
    • l := mid + 1
• return letters[l mod size of letters]

Let us see the following implementation to get better understanding −

Example

 Live Demo

class Solution:
   def nextGreatestLetter(self, letters, target):
      l = 0
      r = len(letters) - 1
      while l <= r:
         mid = (l + r)//2
         if letters[mid] > target:
            r = mid -1
         else:
            l = mid + 1
      return letters[l % len(letters)]
ob = Solution()
print(ob.nextGreatestLetter(["c", "f", "j"], "a"))

Input

["c", "f", "j"], "a"

Output

c