Program to find largest substring between two equal characters in Python

Suppose we have a string s, we have to find the length of the longest substring between two equal letters or elements, excluding the two characters. If we cannot find such substring, then return -1.

So, if the input is like s = "level", then the output will be 3 as optimal substrings can be either "lev" or "vel".

To solve this, we will follow these steps −

• memo := a new map

• for i in range 0 to size of s - 1, do

  • if s[i] is in memo, then

    • insert i at the end of memo[s[i]]

  • otherwise,

    • memo[s[i]] := a list with only one element i

• best := 0

• for each key in memo, do

  • best := maximum of best and (last element of memo[key] - first element of memo[key])

• return best - 1

Example (Python)

 Live Demo

def solve(s):
   memo = {}
   for i in range(len(s)):
      if s[i] in memo:
         memo[s[i]].append(i)
      else:
         memo[s[i]] = [i]

   best = 0
   for key in memo:
      best = max(best, memo[key][-1] - memo[key][0])
   return best - 1

s = "level"
print(solve(s))

Input

"level"

Output

3