Add C++ solution: 420. Strong Password Checker

[anya-minamoto]

Description

Add a C++ solution for LeetCode 420 – Strong Password Checker.

Problem: https://leetcode.com/problems/strong-password-checker/
Language: C++
File path: C++/420-strong-password-checker.cpp

Approach (summary):

• Count missing character classes (lowercase / uppercase / digit).
• Scan for repeating runs of the same char (length ≥ 3) and compute needed replacements (len/3).
• Handle by length:
  • n < 6: need inserts to reach 6 ⇒ answer = max(missing, 6 - n).
  • 6 ≤ n ≤ 20: answer = max(missing, totalReplacements).
  • n > 20: delete del = n - 20 characters; spend deletions to reduce replacements:
    1. Use 1 delete on runs with len % 3 == 0,
    2. then 2 deletes on len % 3 == 1,
    3. then any remaining deletes in groups of 3.
       Final answer = deletions + max(missing, replacementsAfterDeletions).

Time Complexity: O(n)
Space Complexity: O(k) for run tracking (≤ O(n) in worst case)

Difficulty: Hard
Tags: Greedy, Strings

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Put check marks:

• Added problem & solution under correct topic (C++ folder).
• Specified Space & Time complexity.
• Specified difficulty level, tag & notes.

(README change not required by this repo for adding a single solution; if needed I can update it.)

How Has This Been Tested?

• Verified on LeetCode: Accepted (54/54 tests).
• Local sanity checks:
  • "a" → 5
  • "aA1" → 3
  • "ABABABABABABABABABAB1a" → 2
  • No triple repeats + length in [6,20] returns 0.

Checklist

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code so that it is easy to understand.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.

[welcome]

I can tell this is your first pull request! Thank you I'm so honored. 🎉🎉🎉 I'll take a look at it ASAP!