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".

Algorithm

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

Implementation

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))

The output of the above code is −

3

How It Works

The algorithm works by tracking all positions of each character in a dictionary. For each character that appears multiple times, it calculates the distance between its first and last occurrence, then subtracts 1 to exclude the matching characters themselves.

Example with Different Input

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

# Test with different strings
test_cases = ["level", "abcba", "abc", "aa"]

for s in test_cases:
    result = solve(s)
    print(f"String: '{s}' ? Length: {result}")

String: 'level' ? Length: 3
String: 'abcba' ? Length: 3
String: 'abc' ? Length: -1
String: 'aa' ? Length: -1

Conclusion

This solution efficiently finds the longest substring between two equal characters using a dictionary to track character positions. The time complexity is O(n) where n is the length of the string.