7.8 — Interview Questions: Advanced Array Problems

<< Overview

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Q1. Container with most water — explain approach.

Answer: Two pointers from ends. Move the shorter wall inward. O(n).

Q2. Three Sum — how to avoid duplicates?

Answer: Sort array, skip duplicate values for i, l, r.

Q3. Explain prefix sum for subarray sum = K.

Answer: Store cumulative sums in a map. Check if (sum-k) exists. O(n).

Q4. Spiral matrix traversal algorithm?

Answer: Use 4 boundaries (top/bottom/left/right), traverse and shrink.

Q5. How to rotate a matrix 90° in-place?

Answer: Transpose then reverse each row. O(n²) time, O(1) space.

Q6. Search in sorted 2D matrix — optimal approach?

Answer: Treat as 1D sorted array, binary search. O(log(m×n)).

Q7. Set matrix zeroes in O(1) space?

Answer: Use first row and column as markers.

Q8. Minimum window substring approach?

Answer: Variable sliding window with frequency maps. O(n).

Q9. How does the sliding window maximum work?

Answer: Use a deque maintaining indices of potential maximums. O(n).

Q10. Four Sum approach?

Answer: Sort + two nested loops + two pointers. O(n³).

Q11. Next permutation algorithm?

Answer: Find rightmost ascending pair, swap with ceiling, reverse suffix.

Q12. Longest increasing subsequence complexity?

Answer: O(n log n) with patience sorting / binary search approach.

Q13. How to count subarrays with exactly K elements?

Answer: atMost(K) - atMost(K-1) technique.

Q14. Merge intervals algorithm?

Answer: Sort by start, merge overlapping. O(n log n).

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Quick-fire table

#	Question	Answer
1	Container water?	Two pointers, O(n)
2	3Sum time?	O(n²) with sort
3	Prefix sum query?	O(1) after O(n) build
4	Spiral traverse?	4 boundaries
5	Rotate matrix?	Transpose + reverse rows
6	Search sorted 2D?	Binary search O(log mn)
7	Set zeroes space?	O(1) using first row/col
8	Min window?	Variable sliding window
9	Next permutation?	Find, swap, reverse
10	Merge intervals?	Sort by start, merge

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Rapid self-check cards

SC-001

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-002

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-003

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-004

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-005

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-006

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-007

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-008

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-009

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-010

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-011

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-012

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-013

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-014

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-015

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-016

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-017

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-018

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-019

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-020

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-021

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-022

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-023

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-024

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-025

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-026

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-027

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-028

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-029

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-030

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-031

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-032

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-033

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-034

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-035

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-036

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-037

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-038

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-039

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-040

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-041

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-042

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-043

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-044

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-045

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-046

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-047

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-048

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-049

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-050

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-051

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-052

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-053

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-054

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-055

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-056

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-057

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-058

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-059

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-060

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-061

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-062

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-063

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-064

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-065

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-066

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-067

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-068

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-069

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-070

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-071

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-072

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-073

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-074

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-075

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-076

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-077

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-078

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-079

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-080

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-081

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-082

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-083

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-084

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-085

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-086

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-087

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-088

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-089

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-090

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-091

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-092

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-093

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-094

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-095

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-096

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-097

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-098

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-099

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-100

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-101

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-102

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-103

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-104

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-105

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-106

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-107

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-108

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-109

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-110

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-111

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-112

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-113

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-114

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-115

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-116

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-117

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-118

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-119

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-120

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-121

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-122

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-123

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-124

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-125

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-126

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-127

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-128

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-129

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-130

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-131

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-132

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-133

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-134

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-135

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-136

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-137

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-138

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-139

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-140

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-141

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-142

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-143

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-144

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-145

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-146

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-147

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-148

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-149

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-150

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-151

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-152

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-153

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-154

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-155

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-156

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-157

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-158

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-159

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-160

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-161

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-162

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-163

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-164

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-165

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-166

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-167

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-168

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-169

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-170

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-171

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-172

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-173

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-174

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-175

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-176

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-177

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-178

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-179

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-180

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-181

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-182

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-183

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-184

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-185

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-186

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-187

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-188

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-189

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-190

• Q: Merge intervals?
• A: Sort by start, extend or add

SC-191

• Q: Container with most water approach?
• A: Two pointers from ends, move shorter

SC-192

• Q: 3Sum complexity?
• A: O(n²) with sorting

SC-193

• Q: Prefix sum for subarray sum K?
• A: Map of cumulative sums, check sum-k

SC-194

• Q: Spiral order algorithm?
• A: 4 boundaries: top/bottom/left/right

SC-195

• Q: Rotate matrix 90° in-place?
• A: Transpose then reverse each row

SC-196

• Q: Search sorted matrix?
• A: Binary search treating as 1D array

SC-197

• Q: Set zeroes in O(1) space?
• A: Use first row/column as markers

SC-198

• Q: Sliding window maximum?
• A: Deque of indices, O(n)

SC-199

• Q: Next permutation?
• A: Find rightmost ascent, swap, reverse

SC-200

• Q: Merge intervals?
• A: Sort by start, extend or add