Arrays & Strings

Array patterns

Two pointers

# Reverse a list in place\ndef reverse(nums):\n    left, right = 0, len(nums) - 1\n    while left < right:\n        nums[left], nums[right] = nums[right], nums[left]\n        left += 1\n        right -= 1\n    return nums

Sliding window

# Maximum sum of any 3 consecutive numbers\ndef max_window_sum(nums, k):\n    window = sum(nums[:k])\n    best = window\n    for i in range(k, len(nums)):\n        window += nums[i] - nums[i - k]\n        best = max(best, window)\n    return best

String building

def is_palindrome(s: str) -> bool:\n    s = s.lower()\n    cleaned = "".join(c for c in s if c.isalnum())\n    return cleaned == cleaned[::-1]

Frequency counting

def most_frequent(nums):\n    counts = {}\n    for n in nums:\n        counts[n] = counts.get(n, 0) + 1\n    return max(counts, key=counts.get)

The checklist

• Can you avoid nested loops with a hash map?
• Is the data sorted? Two pointers may work
• Do you need all subarrays, or just max/min? (window)
• Could you process from both ends?

Tip: A dict turns a lookup that costs O(n) into one that costs O(1). Most "can you do it in one pass?" answers use a map.