Phase 7Days 36-39

Strings: Basic & Medium

Comfortable string manipulation and the medium classics (heavy algorithms come later)

Phase Goal

Handle strings fluently and solve the standard medium problems before advanced string algorithms.

Progress

Day 36: String fundamentals & easy problems

Reverse words in a string (trim extra spaces)Remove the outermost parenthesesLargest odd number in a string (rightmost odd digit)In-place versus new-string trade-offs

Anagram check with a 26-length frequency arrayIsomorphic strings with two mapsRotate-string check (the concatenation trick)Sort characters by frequency

Longest common prefix across stringsMax nesting depth of parenthesesCareful input reading (getline versus >>)Exact output formatting

Choose one core problem from LeetCode, Codeforces, AtCoder, or CSES after checking its prerequisites; hide its technique labelSpend 10–20 minutes retrieving one earlier mechanism and one earlier bug before new work; prioritize weak prerequisitesUse a five-task bank: guided warm-up, core, changed constraint, delayed recall, optional stretch; attempt one or two, not all five on a timerBefore a solution, produce a trace or brute-force plan; when stuck take one hint, act on it, then reassessRecord help used, the failing input, the corrected reasoning, and a next review date; start around 1/3/7/14/30 days and adaptReserve a weekly mixed review; after foundation readiness add a short virtual contest and upsolve one task with known prerequisitesProve progress through a delayed blank-editor reconstruction and a changed problem; a same-day quiz is supporting evidence only

Day 37: Parsing & number strings

myAtoi: sign, whitespace, overflow clampingRoman numeral to integer (the subtractive rule)Integer to Roman (greedy)Robust parsing habits

Longest palindromic substring by center expansion O(n^2)Odd versus even centersCounting palindromic substringsWhy we defer the O(n) Manacher to advanced strings

Sum of beauty of all substringsFrequency-based scanningSliding character countsRecognizing counting-per-substring shapes

Day 38: Two-pointer on strings

Reverse a string or its vowels with two pointersValid palindrome ignoring non-alphanumericsCase-insensitive comparisonsSkipping invalid characters

Longest substring without repeating charactersCharacter index mapsShrinking the window on a repeatA bridge to the sliding-window phase

Avoiding O(n^2) concatenation in a loopreserve and append patternsstringstream for tokenizingCommon causes of TLE in string problems

Day 39: Strings mixed set & recap

Reverse words II (in place)Sort characters by frequency (heap or count)Group anagrams (hash by a signature)Encode and decode strings (length prefixing)

Turning a string into a comparable signatureSorted-char signature versus count signatureCollisions and when to worryA preview of polynomial hashing (advanced phase)

Choosing frequency array versus map versus two pointersFormat and edge cases (empty, single char)Pattern log updatePrep for linked lists

Phase Complete!

After this phase, you'll be able to:

String fundamentals & easy problems
Parsing & number strings
Two-pointer on strings
Strings mixed set & recap

After Phase 7, explain a core invariant, reconstruct a solution after a delay, diagnose a bug, and solve one unlabelled variation. Repair a failed prerequisite before advancing; record help separately from independent work.

Back to all phases

Strings: Basic & Medium

Phase Goal

Day 36: String fundamentals & easy problems

Manipulating strings

Character counting

Prefix / format problems

Practice, delayed recall, and independent transfer

Day 37: Parsing & number strings

String to number

Palindrome basics

Aggregate / beauty problems

Practice, delayed recall, and independent transfer

Day 38: Two-pointer on strings

In-place two pointers

Substring scanning

String building & efficiency

Practice, delayed recall, and independent transfer

Day 39: Strings mixed set & recap

Medium mix

Hashing strings (intro)

Decision & reflection

Practice, delayed recall, and independent transfer

Phase Complete!