aloalgoIntroduction to Strings
A string is a sequence of characters. In most programming languages, strings are immutable, meaning once created, they cannot be modified. This has important implications for time complexity.String Basics
Why Immutability Matters
Since strings are immutable, concatenation creates a new string each time. Concatenating n single characters one by one is O(n²), not O(n).Common String Operations
Slicing
Searching
Modifying (Creates New String)
Splitting and Joining
Common String Patterns
Pattern 1: Character Frequency
Pattern 2: Two Pointers
Use two pointers moving toward each other for palindrome checks, or moving in the same direction for other patterns.Pattern 3: Sliding Window
Find substrings with certain properties using a sliding window.Pattern 4: String Building
Build strings efficiently using a list and join at the end.Pattern 5: String Matching
Useful String Methods
Time Complexity Reference
| Operation | Time | Notes |
|---|---|---|
| Access s[i] | O(1) | |
| Slice s[i:j] | O(j-i) | Creates new string |
| Concatenation + | O(n+m) | Creates new string |
| join() | O(n) | Total length of strings |
| find/index | O(n*m) | n=text, m=pattern |
| in operator | O(n*m) | Same as find |
| split() | O(n) | |
| replace() | O(n) | Creates new string |
Common Interview Tips
- Remember strings are immutable: Use list for building strings, then join at the end for O(n) instead of O(n²).
- Use Counter for frequency problems: It's cleaner and handles edge cases automatically.
- Consider ASCII values: For character math, use ord() and chr(). E.g., ord('a') - ord('A') = 32.
- Two pointers for palindromes: Compare from both ends moving inward.
- Sliding window for substrings: Maintain a window with certain properties.
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