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4083. Stable Subarrays With Equal Boundary and Interior Sum

My accepted Python solution to LeetCode problem 4083, Stable Subarrays With Equal Boundary and Interior Sum, running in 387ms.

  • Difficulty: Medium
  • Python
  • Runtime 387ms
  • Memory 59.1MB

Read the problem on LeetCode View on GitHub

The problem statement is LeetCode’s and stays on their site. What follows is my accepted solution.

Python

Accepted on LeetCode — runtime 387ms, memory 59.1MB, accepted 2025-12-30.

python
class Solution:
    def countStableSubarrays(self, capacity: List[int]) -> int:
        n = len(capacity)
        count = 0
        prefix = [0] * (n + 1)
        for i in range(n):
            prefix[i + 1] = prefix[i] + capacity[i]
        
        from collections import defaultdict
        seen = defaultdict(int)
        
        for r in range(2, n):  # r starts from 2 (minimum length 3)
            # Add l = r - 2 first (so it can be matched)
            l = r - 2
            seen[(capacity[l], prefix[l])] += 1
            
            # Then check for matches
            target = prefix[r] - 2 * capacity[r]
            count += seen[(capacity[r], target)]
        
        return count

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