intermediate
10 min read
Count and Say
The Count and Say solution is a repeated state transition: start with "1", scan the current string into maximal consecutive runs, and emit each run as…
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State Tracking & Invariants
Track the smallest evolving state needed to preserve a scan or simulation invariant.
Problems
Problems in State Tracking & Invariants
Practice problems that share this primary solution pattern and compare the clues that reveal it.
beginner
10 min read
Find the Index of the First Occurrence in a String
The code is short. The contract is precise: test every legal starting position, require a complete match, and return as soon as the earliest one succeeds.
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beginner
9 min read
Longest Common Prefix
The trap is to compare whole strings before identifying the stopping condition. The answer ends at the first column where agreement breaks—or when the…
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intermediate
11 min read
Longest Palindromic Substring
The difficult part is not recognizing a palindrome. It is preserving contiguity while avoiding repeated work.
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advanced
11 min read
Next Permutation
The reliable way to solve Next Permutation is to stop thinking in four memorized steps. Read the suffix, identify where it is already maximal, then make…
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beginner
10 min read
Palindrome Number
A full reverse is easy to write. The interview-level move is to stop reversing when the two halves meet.
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intermediate
11 min read
Reverse Integer
Reversing digits is easy. Reversing them without ever creating an unsafe intermediate value is the interview problem.
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beginner
10 min read
Roman to Integer
Adding every Roman symbol works for LVIII, but it turns IV into 6. The reliable Roman to Integer solution is a left-to-right scan with one local question:…
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intermediate
10 min read
String to Integer (atoi)
A reliable atoi parser is a small state machine: skip leading spaces, read one optional sign, consume one numeric prefix, and guard every accumulator…
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intermediate
11 min read
Zigzag Conversion
A visual zigzag is easy to draw and surprisingly easy to implement incorrectly. The reliable solution is smaller: track the current row, track the movement…
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intermediate
10 min read
Add Two Numbers
The lists already expose digits in the order addition needs. Scan both lists together, track one carry, and keep going until there is no digit or carry…
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beginner
11 min read
Add Binary
You receive two binary strings, a and b, and must return their sum as another binary string. The inputs contain only '0' and '1', have lengths from 1 to…
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beginner
9 min read
Length of Last Word
The final character may be a space, so the right edge of the string is not necessarily part of the answer. The clean Length of Last Word solution is a…
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intermediate
10 min read
Multiply Strings
Treat the product as a fixed array of decimal positions. Every digit pair has a predictable destination; carry normalization keeps those positions valid.
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advanced
11 min read
Permutation Sequence
Do not generate the permutations. Locate the block, choose its digit, and repeat.
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beginner
9 min read
Plus One
Adding one looks trivial until the last digit is 9. Then the problem becomes a compact state-tracking exercise: start where arithmetic begins, propagate a…
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advanced
12 min read
Text Justification
A line can contain the correct words and still be wrong: one space in the wrong gap, one missing trailing space, or full justification applied to the final…
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advanced
12 min read
Valid Number
A decimal point and an exponent are easy to track. The hard part is proving that every numeric component actually contains the digits the grammar requires.
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