LEVEL 1 OF 5

Foundations

Build your algorithmic intuition.

Arrays

A sequence gives every value an index. First decide what information a single pass can maintain.

Treat text as a sequence, but define what counts as a character before choosing indices.

Trade a little memory for fast answers to: have I seen the value I need?

Remember membership without storing a separate associated value.

Maintain two positions whose movement preserves a useful relationship: mirrored characters, a compacted prefix, or a sorted pair sum.

Maintain a contiguous range, reusing information as its boundaries move.

Store the sum of each prefix so an inclusive range can be expressed as the difference of two prefixes.

Reorder values by a chosen key so local comparisons reveal global structure, such as neighboring overlapping ranges.

A comparison proves that one half of the remaining candidates cannot contain the answer.