leetcode/islands/decks/arrays-and-strings.md

Arrays & Strings

Ordered collections. Arrays are mutable, strings are immutable — every "edit" to a string builds a new one.

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Cost model

Operation	Array	String
Append / pop at end	O(1)*	O(n)
Insert / delete in middle	O(n)	O(n)
Random access	O(1)	O(1)
Membership check	O(n)	O(n)

*amortized — occasional resize, constant on average.

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Recognition

• Contiguous data + index arithmetic → array patterns apply
• Building a string piece by piece → collect parts, join once
• "In-place" requirement → think swap / overwrite, not rebuild

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String building

// O(n^2): each += copies the whole string
let s = "";
for (const ch of parts) s += ch;

// O(n): buffer then join
const buf = [];
for (const ch of parts) buf.push(ch);
const out = buf.join("");

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Recall

• Why is append amortized O(1) and not plain O(1)?
• What does immutability cost when you "modify" one character?
• Which patterns sit on top of arrays? (two pointers, sliding window, prefix sum)

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Drill

• 1480 Running Sum of 1d Array
• 217 Contains Duplicate
• 189 Rotate Array
• 977 Squares of a Sorted Array