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LeetCode Pattern/Prefix Sum/Prefix Sum Edge Cases: Empty Arrays, Single Elements, and All Zeros

Prefix Sum Edge Cases: Empty Arrays, Single Elements, and All Zeros

LeetCopilot Team
Dec 22, 2025
5 min read
Prefix SumEdge CasesTestingInterview Prep
Your solution passes examples but fails edge cases. Learn the comprehensive edge case checklist for prefix sum problems.
Complete Guide

The Complete Prefix Sum Guide: Master All Patterns, Templates, and Techniques

The ultimate comprehensive guide to prefix sum. Learn all variants (1D, 2D, hash map combinations), when to use each pattern, complete templates in multiple languages, and a systematic approach to solve any prefix sum problem.

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Your prefix sum solution works on normal inputs. You submit. Wrong Answer on edge case 3/100.

Sound familiar? Edge cases are where most solutions break. Empty arrays, single elements, all zeros—each needs careful handling.

TL;DR

Critical edge cases:

  1. Empty array
  2. Single element
  3. All zeros
  4. All same values
  5. Target = 0
  6. Negative numbers

Edge Case 1: Empty Array

python
nums = []
# Should return 0 or empty result, not crash

Handling:

python
if not nums:
    return 0  # or [] depending on problem

Edge Case 2: Single Element

python
nums = [5], k = 5
# Should return 1 (the single element)

Trace:

python
prefix = [0, 5]
sum_count = {0: 1, 5: 1}
# Works correctly!

Edge Case 3: All Zeros

python
nums = [0, 0, 0], k = 0
# How many subarrays sum to 0?
# Answer: 6 (all possible subarrays)

Calculation:

  • [0] at index 0, 1, 2: 3 subarrays
  • [0,0] at indices (0,1), (1,2): 2 subarrays
  • [0,0,0]: 1 subarray
  • Total: 6

Edge Case 4: All Same Values

python
nums = [1, 1, 1], k = 2
# [1,1] appears twice

Edge Case 5: Target = 0

python
nums = [1, -1, 0], k = 0
# Subarrays: [1,-1], [0], [1,-1,0]
# Count: 3

Defensive Coding Checklist

python
def subarraySum(nums, k):
    # 1. Handle empty array
    if not nums:
        return 0
    
    # 2. Initialize correctly
    count = 0
    prefix_sum = 0
    sum_count = {0: 1}  # Don't forget!
    
    # 3. Main logic
    for num in nums:
        prefix_sum += num
        if prefix_sum - k in sum_count:
            count += sum_count[prefix_sum - k]
        sum_count[prefix_sum] = sum_count.get(prefix_sum, 0) + 1
    
    return count

Test Cases

python
# Test all edge cases
assert subarraySum([], 0) == 0
assert subarraySum([1], 1) == 1
assert subarraySum([0,0,0], 0) == 6
assert subarraySum([1,1,1], 2) == 2
assert subarraySum([1,-1,0], 0) == 3

Conclusion

Key takeaway: Always test edge cases before submitting.

For more, see the complete guide and debugging guide.

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