LeetCode: Second Largest Digit in a String

Problem:

https://leetcode.com/problems/second-largest-digit-in-a-string/

Code:

class Solution:
    def secondHighest(self, s: str) -> int:
        dgts = set()
        for ch in s:
            if ch.isdigit():
                dgts.add(int(ch)) 
        # now we have all unique digits

        # if dgts has 1 or no value, there can be no 2nd largest
        if len(dgts)<=1:
            return -1

        # if control come here, it has sufficient values
        # remove the max digit
        dgts.remove(max(dgts))
        return max(dgts)

# class Solution:
#     def secondHighest(self, s: str) -> int:
#         dgts = []
#         for ch in s:
#             if ch.isdigit():
#                 dgts.append(int(ch))

#         if not dgts:
#             return -1

#         dgts = set(dgts)
#         dgts.remove(max(dgts))

#         if len(dgts)<1:
#             return -1
#         else:
#             return max(dgts)

Key Points:

Problem in Simple Words

You are given a string that may contain letters and digits. You need to find the second largest digit that appears in the string.

• Digits are from 0 to 9

• Digits must be distinct

• If there is no second largest digit → return -1

Example:

• "dfa12321afd" → digits = {1,2,3} → second largest = 2

• "abc111" → only one digit → return -1

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Core Idea (Use a Set for Unique Digits)

We only care about:

• digits, not letters

• unique digits, not duplicates

So the best structure is a set:

• Automatically removes duplicates

• Makes it easy to find max values

---

How the Solution Works

1. Traverse the string character by character

2. If the character is a digit:

   • Convert it to int

   • Add it to a set

3. After traversal:

   • If the set has 0 or 1 element → no second largest → return -1

4. Otherwise:

   • Remove the largest digit

   • The new maximum is the second largest digit

   • Return it

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Why Removing the Maximum Works

• After removing the largest digit,

• the largest remaining digit is automatically the second largest overall

• No sorting is needed

---

Why This Approach Is Efficient

• Set ensures uniqueness without extra checks

• We scan the string only once

• Operations like max() on a small set (digits 0–9) are very fast

---

Edge Cases Covered

• No digits in string → return -1

• Only one unique digit → return -1

• Multiple same digits → handled by set

---

Complexity

• Time: O(n), where n = length of string

• Space: O(1), because at most 10 digits (0–9) can be stored

---

Key Insight to Remember

Whenever a problem asks for:

“largest / second largest unique value”

👉 Think set + remove max, instead of sorting everything.