7.5 — Quick Revision: Strings

<< Overview

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String methods cheat sheet (JS)

Method	Returns	Mutates?
concat()	new string	No
slice(s,e)	substring	No
substring(s,e)	substring	No
replace()	new string (first match)	No
replaceAll()	new string (all matches)	No
indexOf()	index or -1	No
includes()	boolean	No
startsWith()	boolean	No
toUpperCase()	new string	No
trim()	new string	No
split()	array	No
repeat(n)	new string	No
padStart(n,c)	new string	No

Key algorithms

Algorithm	Time	Space	Pattern
Reverse	O(n)	O(n) or O(1)	Two pointers
Palindrome	O(n)	O(1)	Two pointers
Anagram check	O(n)	O(1)*	Frequency map
Longest unique substr	O(n)	O(k)	Sliding window
LCP	O(S)	O(1)	Vertical scan
KMP matching	O(n+m)	O(m)	LPS array
Edit distance	O(mn)	O(mn)	DP table

Common pitfalls

1. Strings are immutable in JS — methods return new strings
2. == compares value for primitives, reference for objects
3. indexOf returns -1 (not false) when not found
4. Template literals use backticks, not quotes
5. split("") creates array of chars; split() creates single-element array

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Self-check drill

SC-001

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-002

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-003

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-004

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-005

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-006

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-007

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-008

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-009

• Q: What does split('') do?
• A: Creates array of individual characters

SC-010

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-011

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-012

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-013

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-014

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-015

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-016

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-017

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-018

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-019

• Q: What does split('') do?
• A: Creates array of individual characters

SC-020

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-021

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-022

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-023

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-024

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-025

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-026

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-027

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-028

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-029

• Q: What does split('') do?
• A: Creates array of individual characters

SC-030

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-031

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-032

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-033

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-034

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-035

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-036

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-037

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-038

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-039

• Q: What does split('') do?
• A: Creates array of individual characters

SC-040

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-041

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-042

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-043

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-044

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-045

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-046

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-047

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-048

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-049

• Q: What does split('') do?
• A: Creates array of individual characters

SC-050

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-051

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-052

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-053

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-054

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-055

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-056

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-057

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-058

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-059

• Q: What does split('') do?
• A: Creates array of individual characters

SC-060

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-061

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-062

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-063

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-064

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-065

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-066

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-067

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-068

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-069

• Q: What does split('') do?
• A: Creates array of individual characters

SC-070

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-071

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-072

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-073

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-074

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-075

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-076

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-077

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-078

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-079

• Q: What does split('') do?
• A: Creates array of individual characters

SC-080

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-081

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-082

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-083

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-084

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-085

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-086

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-087

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-088

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-089

• Q: What does split('') do?
• A: Creates array of individual characters

SC-090

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-091

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-092

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-093

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-094

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-095

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-096

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-097

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-098

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-099

• Q: What does split('') do?
• A: Creates array of individual characters

SC-100

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-101

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-102

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-103

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-104

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-105

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-106

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-107

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-108

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-109

• Q: What does split('') do?
• A: Creates array of individual characters

SC-110

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-111

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-112

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-113

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-114

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-115

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-116

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-117

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-118

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-119

• Q: What does split('') do?
• A: Creates array of individual characters

SC-120

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-121

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-122

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-123

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-124

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-125

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-126

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-127

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-128

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-129

• Q: What does split('') do?
• A: Creates array of individual characters

SC-130

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-131

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-132

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-133

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-134

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-135

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-136

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-137

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-138

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-139

• Q: What does split('') do?
• A: Creates array of individual characters

SC-140

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-141

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-142

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-143

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-144

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-145

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-146

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-147

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-148

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-149

• Q: What does split('') do?
• A: Creates array of individual characters

SC-150

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-151

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-152

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-153

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-154

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-155

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-156

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-157

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-158

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-159

• Q: What does split('') do?
• A: Creates array of individual characters

SC-160

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-161

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-162

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-163

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-164

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-165

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-166

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-167

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-168

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-169

• Q: What does split('') do?
• A: Creates array of individual characters

SC-170

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-171

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-172

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-173

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-174

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-175

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-176

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-177

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-178

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-179

• Q: What does split('') do?
• A: Creates array of individual characters

SC-180

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-181

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-182

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-183

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-184

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-185

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-186

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-187

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-188

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-189

• Q: What does split('') do?
• A: Creates array of individual characters

SC-190

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars

SC-191

• Q: How to reverse a string?
• A: split('').reverse().join('') or two pointers

SC-192

• Q: Palindrome check approach?
• A: Two pointers from both ends, compare chars

SC-193

• Q: Anagram check - optimal?
• A: Frequency map in O(n)

SC-194

• Q: Are JS strings mutable?
• A: No — all operations create new strings

SC-195

• Q: KMP time complexity?
• A: O(n+m) — linear

SC-196

• Q: Sliding window for unique chars?
• A: Set + left/right pointers

SC-197

• Q: Edit distance approach?
• A: 2D DP table, compare prefixes

SC-198

• Q: slice vs substring?
• A: slice supports negative, substring swaps args

SC-199

• Q: What does split('') do?
• A: Creates array of individual characters

SC-200

• Q: String compression algorithm?
• A: Run-length encoding: count consecutive chars