Sliding Window: Distinct Characters and Basket Patterns

Learning Outcome

Use a moving window when subarrays or substrings must remain valid under a local constraint.

Pattern Recognition

Item	Detail
Core signal	The problem asks for longest, shortest, or count of contiguous ranges with a condition.
Use when	The answer is contiguous and you can restore validity by moving the left boundary.
Avoid when	The required invariant is not monotonic or the input constraints point to a simpler direct scan.

Intuition

The right pointer explores new data; the left pointer removes old data until the window becomes valid again.

Exact Practice Question Names

Longest Substring Without Repeating Characters
Fruit Into Baskets
Minimum Window Substring
Permutation in String
K-divisible Elements Subarrays

Interview Approach

Expand right and update frequency state.
While invalid, remove left and move left forward.
Update best answer only when the window is valid.
Use last-seen index optimization for no-repeat strings.

Pseudocode

left = 0
state = empty map
for right in range(n):
  add a[right] to state
  while window is invalid:
    remove a[left] from state
    left += 1
  update answer from right - left + 1

Sample Dry Run

For 'abcabcbb', the window grows to 'abc'. Seeing the next 'a' moves left after the old 'a', preserving a duplicate-free window.

Edge Cases

Empty string
All same characters
Constraint k = 0
Zero-count keys left in the map

Common Mistakes

Updating answer before shrinking invalid windows
Using sliding window when negative numbers break monotonicity
Forgetting to delete zero-frequency keys

Complexity

Item	Detail
Expected time	O(n) because each pointer moves at most n times.
Expected space	O(k) for the tracked alphabet or distinct values.

Java, C++ and Python Notes

Java: prefer explicit classes and clear helper methods over clever one-liners.
C++: use vector, unordered_map, set, priority_queue, and long long when sums can grow.
Python: keep state readable with dict, set, deque, heapq, and lru_cache where appropriate.

Quick Revision Checklist

Name the pattern before coding.
State the invariant or DP state in one sentence.
Dry-run the smallest non-trivial example.
Close with time and space complexity.