Easy
Isomorphic Strings:Map Both Directions, Not One
s maps onto t when each letter has one partner and that partner has only s. A single HashMap hides collisions the other way; two maps (or two last-seen indexes) catch them.
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21 questions
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- Easy
Longest Common Prefix:Walk Columns, Stop at the First Mismatch
The shared prefix of a list of strings is the first column that disagrees. Pairwise shrinks waste work; one vertical scan stops at the first mismatch.
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Longest Palindrome:Pair the Evens, Spend One Odd in the Center
The longest palindrome you can build from letter counts is every even pair plus at most one odd center. This is not the longest palindromic substring of the original string.
Read More - Easy
Roman to Integer:Subtract When the Smaller Numeral Comes First
A Roman string is a left-to-right sum unless a smaller numeral sits before a larger one. Nested replace of IV/IX pairs works; one peek at the next value is enough.
Read More - Easy
Valid Anagram:Count the Letters, Do Not Sort Both Copies
Two strings are anagrams when their letter frequencies match. Sorting both is honest and slower; one count array (or a map) answers in linear time.
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Valid Palindrome:Skip the Noise, Meet in the Middle
A phrase is a palindrome after you ignore punctuation and case. Building a cleaned copy works; two pointers from the ends answer without the extra string.
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- Medium
Decode Ways:One Digit or Two, Skip the Leading Zero
A digit string decodes as letters 1–26. Recursing on every split is exponential; a linear pass counts ways to land on i from a legal one-digit or two-digit tail.
Read More - Medium
Edit Distance:Insert, Delete, or Replace on a Costed Grid
Turning word1 into word2 is the cheapest insert, delete, or substitute at each prefix pair. Recursing on three ops explodes; a DP grid fills each cell from three neighbors.
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Find All Anagrams in a String:Collect Every Matching Window, Not the First
Every anagram of p inside s is a window of the same length whose counts match. Rechecking each slice is quadratic; one sliding frequency pass returns every start index.
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Group Anagrams:Hash the Signature, Not Every Pair of Words
Anagrams share a letter signature. Pairwise checks are honest and quadratic; one map from sorted letters (or a count key) to a list groups them in a pass.
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Letter Combinations of a Phone Number:Choose a Letter, Recurse, Undo
Each digit opens a keypad bucket. Nested loops freeze the length; backtracking walks the digits, appends one letter, and undoes so the next choice sees a clean path.
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Longest Common Subsequence:Match or Skip, Do Not Require Contiguous
The longest shared subsequence of two strings is a DP grid: match both letters or skip one. Enumerating subsequences is exponential; the grid answers in O(mn) without requiring a contiguous slice.
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Longest Palindromic Substring:Expand From the Center, Do Not Test Every Slice
The longest palindrome sitting inside the string is found by expanding around 2n−1 centers. Checking every i..j is honest and cubic; a DP table matches the O(n²) bound with extra space you do not need.
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Longest Repeating Character Replacement:The Window Can Spare k Mismatches
The longest same-letter stretch after at most k replacements is a window whose length minus the majority count stays ≤ k. Nested slices are honest; one grow/shrink pass is enough.
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Longest Substring Without Repeating Characters:Slide Past the Repeat, Do Not Restart
The longest unique-character stretch is a window that grows on the right and jumps the left past a repeat. Nested uniqueness checks are honest and quadratic.
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Palindrome Partitioning:Cut Only When the Prefix Is a Palindrome
Every partition of s into palindromic slices is a cut that is legal only if the prefix is already a palindrome. Nested splits explode; backtracking tries each end, undoes, and continues.
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Palindromic Substrings:Count Every Expansion, Not Only the Longest
Every palindromic window counts, including single letters. Nested i..j checks are honest; expanding around 2n−1 centers is the same walk as longest palindromic substring, with a counter instead of a best slice.
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Permutation in String:Slide a Fixed Window, Do Not Recheck Every Anagram
s2 contains a permutation of s1 when some window of length s1 matches its letter counts. Sorting every slice is honest; one sliding count answers in linear time.
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Reverse Words in a String:Flip the Sentence, Then Flip Each Word
Word order reverses after you drop extra spaces. split and reverse a list is honest; reversing the whole buffer then each word does the same job without a token array.
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Word Break:Mark Every Index a Dictionary Word Can Reach
s segments if some dictionary words cover it end to end. Recursing on every prefix is exponential; a boolean array records which indexes a word can land on, and a set answers membership.
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