LeetCode Pattern/Hash Map/Why Your Subarray Sum Equals K Solution Fails (3 Common Bugs)

Why Your Subarray Sum Equals K Solution Fails (3 Common Bugs)

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Dec 22, 2025

7 min read

Hash MapPrefix SumDebuggingCommon Mistakes

Getting 'Wrong Answer' on Subarray Sum Equals K? Learn the 3 bugs that trip up 90% of developers and how to fix them instantly.

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You're solving "Subarray Sum Equals K." You write what looks like a perfect prefix sum + hash map solution.

You submit. Wrong Answer.

Test case 23 fails. You can't figure out why.

This is frustrating because the pattern seems straightforward. But there are 3 specific bugs that trip up 90% of developers on this problem.

This guide shows you exactly what they are and how to fix them.

Bug 1: Forgetting {0: 1} Initialization

The Problem

This is the #1 most common mistake on Subarray Sum Equals K.

❌ Wrong (missing initialization):

python

def subarraySum(nums, k):
    count = 0
    prefix_sum = 0
    sum_count = {}  # Missing {0: 1}!
    
    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

Why It Fails

Test case: nums = [1, 2, 3], k = 3

Expected: 2 (subarrays [1, 2] and [3])

Trace without {0: 1}:

text

i=0: prefix=1, check 1-3=-2 (not found), add {1: 1}, count=0
i=1: prefix=3, check 3-3=0 (not found!), add {1: 1, 3: 1}, count=0
i=2: prefix=6, check 6-3=3 (found!), count=1, add {1: 1, 3: 1, 6: 1}

Result: 1 ❌ (should be 2)

What we missed: Subarray [1, 2] (sum = 3)

The Fix

✅ Correct (with initialization):

python

def subarraySum(nums, k):
    count = 0
    prefix_sum = 0
    sum_count = {0: 1}  # Critical!
    
    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

Trace with {0: 1}:

text

i=0: prefix=1, check 1-3=-2 (not found), add {0: 1, 1: 1}, count=0
i=1: prefix=3, check 3-3=0 (found!), count=1, add {0: 1, 1: 1, 3: 1}
i=2: prefix=6, check 6-3=3 (found!), count=2, add {0: 1, 1: 1, 3: 1, 6: 1}

Result: 2 ✓

Why {0: 1} Is Needed

Conceptual meaning: "There's one way to get sum 0: take nothing (empty prefix)."

Practical effect: Handles subarrays that start from index 0.

When prefix_sum = k, we need prefix_sum - k = 0 to exist in the map.

The rule: For any prefix sum + hash map problem, initialize with {0: 1}.

Bug 2: Wrong Order of Operations

The Problem

Updating the map before checking causes double-counting.

❌ Wrong (update before check):

python

def subarraySum(nums, k):
    count = 0
    prefix_sum = 0
    sum_count = {0: 1}
    
    for num in nums:
        prefix_sum += num
        
        # Wrong order!
        sum_count[prefix_sum] = sum_count.get(prefix_sum, 0) + 1
        if prefix_sum - k in sum_count:
            count += sum_count[prefix_sum - k]
    
    return count

Why It Fails

Test case: nums = [1], k = 0

Expected: 0 (no subarray sums to 0)

Trace with wrong order:

text

i=0: prefix=1
     Add {0: 1, 1: 1}
     Check 1-0=1 (found!), count=1

Result: 1 ❌ (should be 0)

What went wrong: We added prefix_sum = 1 to the map, then immediately found it when checking for prefix_sum - k = 1. This counts the current element as a subarray by itself, which is wrong when k = 0.

The Fix

✅ Correct (check before update):

python

def subarraySum(nums, k):
    count = 0
    prefix_sum = 0
    sum_count = {0: 1}
    
    for num in nums:
        prefix_sum += num
        
        # Correct order: check FIRST
        if prefix_sum - k in sum_count:
            count += sum_count[prefix_sum - k]
        
        # Then update
        sum_count[prefix_sum] = sum_count.get(prefix_sum, 0) + 1
    
    return count

Trace with correct order:

text

i=0: prefix=1
     Check 1-0=1 (not found), count=0
     Add {0: 1, 1: 1}

Result: 0 ✓

The rule: Always check before updating the map.

Bug 3: Misunderstanding Negative Numbers

The Problem

Thinking you can use sliding window because "it's a subarray problem."

❌ Wrong (trying sliding window):

python

def subarraySum(nums, k):
    # Trying to use sliding window
    left = 0
    current_sum = 0
    count = 0
    
    for right in range(len(nums)):
        current_sum += nums[right]
        
        while current_sum > k:  # When to shrink?
            current_sum -= nums[left]
            left += 1
        
        if current_sum == k:
            count += 1
    
    return count

Why It Fails

Test case: nums = [1, -1, 1, 1], k = 2

Expected: 3 (subarrays [1, -1, 1, 1], [1, 1], [1, 1])

Why sliding window fails: With negative numbers, shrinking doesn't guarantee the sum decreases. You lose the monotonicity property that sliding window requires.

Example:

text

At right=2: sum=1, should we shrink?
If we shrink, we might miss valid subarrays!

The Fix

✅ Correct (use prefix sum + hash map):

python

def subarraySum(nums, k):
    count = 0
    prefix_sum = 0
    sum_count = {0: 1}
    
    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

The rule: If array has negative numbers or you need exact sum, use prefix sum + hash map, not sliding window.

The Correct Template

Here's the bug-free template:

python

def subarraySum(nums: List[int], k: int) -> int:
    """
    Count subarrays with sum equals k.
    
    Time: O(n)
    Space: O(n)
    """
    count = 0
    prefix_sum = 0
    sum_count = {0: 1}  # Bug 1: Must initialize!
    
    for num in nums:
        prefix_sum += num
        
        # Bug 2: Check BEFORE updating
        if prefix_sum - k in sum_count:
            count += sum_count[prefix_sum - k]
        
        # Update after checking
        sum_count[prefix_sum] = sum_count.get(prefix_sum, 0) + 1
    
    return count

Debugging Checklist

If your solution fails, check:

1. Initialization

Did you initialize with {0: 1}?
Not {} or {0: 0}?

2. Order of Operations

Are you checking before updating?
Not updating then checking?

3. Pattern Choice

Are you using prefix sum + hash map?
Not trying sliding window with negative numbers?

4. Formula

Are you checking prefix_sum - k?
Not k - prefix_sum?

5. Update Logic

Are you incrementing count: sum_count.get(prefix_sum, 0) + 1?
Not setting to 1?

Common Test Cases

Test your solution with these:

python

# Test 1: Basic
nums = [1, 2, 3], k = 3
# Expected: 2 ([1,2] and [3])

# Test 2: From start
nums = [1, 1, 1], k = 2
# Expected: 2 ([1,1] at indices 0-1 and 1-2)

# Test 3: Negative numbers
nums = [1, -1, 1, 1], k = 2
# Expected: 3

# Test 4: k = 0
nums = [1], k = 0
# Expected: 0

# Test 5: All zeros
nums = [0, 0, 0], k = 0
# Expected: 6 (all subarrays)

# Test 6: Single element equals k
nums = [3], k = 3
# Expected: 1

Quick Fixes

Fix 1: Missing {0: 1}

python

# Before
sum_count = {}

# After
sum_count = {0: 1}

Fix 2: Wrong order

python

# Before
sum_count[prefix_sum] = sum_count.get(prefix_sum, 0) + 1
if prefix_sum - k in sum_count:
    count += sum_count[prefix_sum - k]

# After
if prefix_sum - k in sum_count:
    count += sum_count[prefix_sum - k]
sum_count[prefix_sum] = sum_count.get(prefix_sum, 0) + 1

Fix 3: Using sliding window

python

# Before
# Sliding window code...

# After
# Use prefix sum + hash map template

Summary

The 3 bugs that cause "Wrong Answer" on Subarray Sum Equals K:

Missing {0: 1} → Misses subarrays from start
Wrong order → Double-counts or misses subarrays
Using sliding window → Fails with negative numbers

The correct template:

python

count = 0
prefix_sum = 0
sum_count = {0: 1}  # Critical!

for num in nums:
    prefix_sum += num
    if prefix_sum - k in sum_count:  # Check first
        count += sum_count[prefix_sum - k]
    sum_count[prefix_sum] = sum_count.get(prefix_sum, 0) + 1  # Update after

return count

The debugging process:

Check initialization: {0: 1}?
Check order: Check before update?
Check pattern: Prefix sum, not sliding window?

Next steps:

Fix these 3 bugs, and Subarray Sum Equals K becomes solvable.

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Why Your Subarray Sum Equals K Solution Fails (3 Common Bugs)

Hash Map Pattern: Complete Guide With Templates (Python/Java/JS)

Table of Contents

Bug 1: Forgetting {0: 1} Initialization

The Problem

Why It Fails

The Fix

Why {0: 1} Is Needed

Bug 2: Wrong Order of Operations

The Problem

Why It Fails

The Fix

Bug 3: Misunderstanding Negative Numbers

The Problem

Why It Fails

The Fix

The Correct Template

Debugging Checklist

1. Initialization

2. Order of Operations

3. Pattern Choice

4. Formula

5. Update Logic

Common Test Cases

Quick Fixes

Fix 1: Missing {0: 1}

Fix 2: Wrong order

Fix 3: Using sliding window

Summary

Want to Practice LeetCode Smarter?

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