Stage 1 · Code
Arrays & Strings
Sliding Window
Fixed and variable-size windows for subarray/substring problems.
7 min readMastering Data Structures & Algorithms for Software Engineering InterviewsCode
Fixed Size Window
Fixed window sliding — find max sum subarray of size k=3
Sliding Window — size k=3
2l
1
5r
1
3
2
7
4
0
1
2
3
4
5
6
7
Window sum:8Best:8
Step 1 / 7 — First window [0..2] = 8
Fixed window of size 3 sliding across the array
2
1
5l
1
3r
2
0
1
2
3
4
5
window [2..4] = 5+1+3 = 9 (maximum)
Window size k is constant. Slide by adding right element, removing left element. Maintain window state (sum, count, frequency). O(n) time.
Gomaximum-sum-subarray-of-size-k.go
23 linesLn 1, Col 1Go
O(n) by reusing previous sum. Subtract element leaving window, add element entering.
Variable Size Window
Window expands (right++) until constraint violated, then shrinks (left++) until valid. Used for 'longest/shortest subarray satisfying condition'.
Golongest-substring-without-repeating-characters.go
23 linesLn 1, Col 1Go
Expand right. When duplicate found in window, jump left past previous occurrence. Track max window size.
Sliding Window Template
Gogeneral-variable-window-template.go
21 linesLn 1, Col 1Go
Core pattern: expand right, shrink left while invalid, update answer. State depends on problem (sum, frequency map, distinct count, etc.).
Classic Examples
| Problem | Window Type | State | Key Insight |
|---|---|---|---|
| Max sum subarray size k | Fixed | Sum | Subtract left, add right |
| Min subarray sum ≥ target | Variable | Sum | Shrink when sum ≥ target |
| Longest substring ≤ k distinct | Variable | Char freq map | Shrink when distinct > k |
| Longest substring no repeat | Variable | Char last index | Jump left to last index + 1 |
| Permutation in string | Fixed | Char freq diff | Match target freq |
| Min window substring | Variable | Char freq + count | Shrink when all chars covered |
| Count subarrays sum = k | Variable | Prefix sum + hashmap | Count prefix[j] - prefix[i] = k |
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