Data structures & algorithms

How the judge runs your code, the read -> compute -> print habit behind every problem, and how to start seeing reusable patterns.

Your first submissions: read a value or two off stdin, return the answer, and match the output format exactly.

Integer types and overflow, the five arithmetic operators, integer versus real division, and turning a sentence into a formula.

Make decisions with if/else and comparisons, chain cases like sign and grade, pick min/max, and combine conditions for rules like leap year.

Repeat work with for/while, then master the running total, running best, and conditional count patterns that power the rest of the ladder.

The nested-loops-draw-a-grid pattern: build each row, join with newlines, and shape triangles and pyramids by tying the inner loop to the row index.

Peel digits with % and /, reverse numbers and test palindromes, find divisors, check primes by trial division, and compute gcd with Euclid.

Treat a string as a sequence of characters: measure length, loop over it, count vowels, reverse it, test palindromes, and check case.

Read a count N then a list of numbers, aggregate it in one pass (sum, max, min), search it linearly, and reverse or count within it.

Read an R×C grid and traverse it with two indices: row and column sums, the two diagonals, and the transpose.

A function that calls itself: always a base case plus a smaller subproblem. Build factorial, sum-to-N, power, and Fibonacci from that one shape.

Three reusable shapes that unlock the interview ladder: frequency count, prefix sum, and two pointers — plus how to recognize which one a problem needs.