Practice Coding Problems

Frequency scan · Index in O(1)

In-place compact · Index in O(1)

In-place swap · Index in O(1)

Slow-fast write pointer · Index in O(1)

Reverse three times · Index in O(1)

Prefix/suffix products · Index in O(1)

Run-length encoding · Index in O(1)

Boyer-Moore voting · Index in O(1)

Index as hash · Index in O(1)

Character-by-character scan · Index in O(1)

Compact != val · Index in O(1)

Collapse consecutive runs · Index in O(1)

Hash-map complement · Sets and maps that answer "seen it?" in O(1)

Character counts · Sets and maps that answer "seen it?" in O(1)

Sorted-string key · Sets and maps that answer "seen it?" in O(1)

Counting + bucket sort · Sets and maps that answer "seen it?" in O(1)

Set + streak from starts · Sets and maps that answer "seen it?" in O(1)

Row/col/box sets · Sets and maps that answer "seen it?" in O(1)

Prefix sums in a map · Sets and maps that answer "seen it?" in O(1)

Pair-sum map · Sets and maps that answer "seen it?" in O(1)

Count then scan · Sets and maps that answer "seen it?" in O(1)

Length-prefix encoding · Sets and maps that answer "seen it?" in O(1)

Two maps or one map plus used-set · Sets and maps that answer "seen it?" in O(1)

Set membership · Sets and maps that answer "seen it?" in O(1)

Inward pointers · Two indexes moving inward

Opposite-end pointers · Two indexes moving inward

Sort + two pointers · Two indexes moving inward

Greedy shrink from ends · Two indexes moving inward

Left/right max pointers · Two indexes moving inward

Three-way partition · Two indexes moving inward

Merge from ends · Two indexes moving inward

Skip one mismatch · Two indexes moving inward

Sort + nested two pointers · Two indexes moving inward

Heaviest with lightest · Two indexes moving inward

Swap vowels from both ends · Two indexes moving inward

Advance s inside t · Two indexes moving inward

Push open, pop on close · Last in, first out

Parallel min stack · Last in, first out

Stack of operands · Last in, first out

Next greater element · Last in, first out

Time-to-target stack · Last in, first out

Nearest smaller bars · Last in, first out

Stack simulate collisions · Last in, first out

Stack of counts and strings · Last in, first out

Next greater in nums2 · Last in, first out

Operator stack / running term · Last in, first out

Pop when top matches · Last in, first out

Circular next greater · Last in, first out

Running minimum · A live range over a sequence

Window + last-seen index · A live range over a sequence

Window with k replacements · A live range over a sequence

Fixed anagram window · A live range over a sequence

Shrink while valid · A live range over a sequence

Monotonic deque · A live range over a sequence

Window with k zeros · A live range over a sequence

Window with 2 kinds · A live range over a sequence

Rolling anagram window · A live range over a sequence

Grow/shrink positive window · A live range over a sequence

Window sum of length k · A live range over a sequence

Alternating greater/less · A live range over a sequence

Merge from the back · Sort once so pair questions need only neighbors

N log n sort · Sort once so pair questions need only neighbors

Sort by concatenation · Sort once so pair questions need only neighbors

Sort or count citations · Sort once so pair questions need only neighbors

Quickselect / heap · Sort once so pair questions need only neighbors

Order map · Sort once so pair questions need only neighbors

Sort and take evens · Sort once so pair questions need only neighbors

Merge sort on lists · Sort once so pair questions need only neighbors

Order then leftovers · Sort once so pair questions need only neighbors

Sort and check overlap · Sort once so pair questions need only neighbors

Count mismatches vs sorted · Sort once so pair questions need only neighbors

Rank unique sorted values · Sort once so pair questions need only neighbors

Classic binary search · Halve a sorted range

Lower bound · Halve a sorted range

Binary search the first true · Halve a sorted range

Binary search over rows*cols · Halve a sorted range

Pivot binary search · Halve a sorted range

Identify sorted half · Halve a sorted range

Binary search eating speed · Halve a sorted range

Walk uphill via mid · Halve a sorted range

Binary search capacity · Halve a sorted range

Binary search partition · Halve a sorted range

Search the peak on a unimodal array · Halve a sorted range

Start top-right, drop a row or column · Halve a sorted range

Iterative reverse · Rewire next links in place

Dummy head merge · Rewire next links in place

Floyd cycle finding · Rewire next links in place

Slow/fast mid · Rewire next links in place

Gap of n pointers · Rewire next links in place

Mid + reverse + merge · Rewire next links in place

Digit-wise carry · Rewire next links in place

Reverse second half · Rewire next links in place

Switch heads to align · Rewire next links in place

Reverse exact groups · Rewire next links in place

Skip nodes equal to val · Rewire next links in place

Odd indices then even indices · Rewire next links in place

Trust the recursive call · Trust a smaller call down to a base case

Fast exponentiation · Trust a smaller call down to a base case

Swap then recurse inward · Trust a smaller call down to a base case

Recurse into nested arrays · Trust a smaller call down to a base case

Row from the row above · Trust a smaller call down to a base case

Swap then recurse on rest · Trust a smaller call down to a base case

Recurse into a half · Trust a smaller call down to a base case

Sum digits until one remains · Trust a smaller call down to a base case

Run-length of previous term · Trust a smaller call down to a base case

Draw the call tree · Trust a smaller call down to a base case

N % 3 == 0 then recurse · Trust a smaller call down to a base case

Row from the previous row · Trust a smaller call down to a base case

Swap children recursively · A node, a left child, a right child

1 + max(left, right) · A node, a left child, a right child

Compare structure and values · A node, a left child, a right child

Height plus longest through node · A node, a left child, a right child

Height-balanced check · A node, a left child, a right child

Queue BFS · A node, a left child, a right child

Subtract along a path · A node, a left child, a right child

Last node per level · A node, a left child, a right child

Return node if found in subtree · A node, a left child, a right child

Gain from one child vs path through · A node, a left child, a right child

Left mirrors right · A node, a left child, a right child

Shortest root-to-leaf · A node, a left child, a right child

Go left or right · Left subtree smaller, right subtree larger

Carry min/max bounds · Left subtree smaller, right subtree larger

Walk until split · Left subtree smaller, right subtree larger

Inorder kth · Left subtree smaller, right subtree larger

Mid as root · Left subtree smaller, right subtree larger

Walk to a leaf · Left subtree smaller, right subtree larger

Successor splice · Left subtree smaller, right subtree larger

Skip branches outside [low, high] · Left subtree smaller, right subtree larger

Controlled inorder stack · Left subtree smaller, right subtree larger

Catalan numbers · Left subtree smaller, right subtree larger

Adjacent inorder values · Left subtree smaller, right subtree larger

Hash complements while walking · Left subtree smaller, right subtree larger

Smash two heaviest · A complete tree in an array

K smallest distances · A complete tree in an array

Size-k min heap · A complete tree in an array

Idle slots from most frequent · A complete tree in an array

Heap of heads · A complete tree in an array

Max heap + min heap · A complete tree in an array

Min heap with lexical ties · A complete tree in an array

Never place the same letter adjacent · A complete tree in an array

Merge 2/3/5 streams · A complete tree in an array

Size-k min heap · A complete tree in an array

Min-heap of candidates · A complete tree in an array

Ladders on the largest climbs · A complete tree in an array

Decision tree include/skip · Choose, explore, unchoose

Used-mask permutations · Choose, explore, unchoose

Reuse the same candidate · Choose, explore, unchoose

Each number once, skip dups · Choose, explore, unchoose

Power set with duplicates · Choose, explore, unchoose

Map digits to letters · Choose, explore, unchoose

Open/close counts · Choose, explore, unchoose

DFS with visit marks · Choose, explore, unchoose

Partition when prefix is palindrome · Choose, explore, unchoose

Place with col/diag attacks · Choose, explore, unchoose

Backtrack choose / skip · Choose, explore, unchoose

K distinct digits 1-9 summing to n · Choose, explore, unchoose

Children map + end flag · A tree of prefixes

DFS on '.' · A tree of prefixes

Replace with shortest prefix in the dict · A tree of prefixes

Word whose every prefix is in the dict · A tree of prefixes

Store value, sum by prefix · A tree of prefixes

Search words one edit away · A tree of prefixes

Bitwise trie · A tree of prefixes

Suffix#word in a trie · A tree of prefixes

DFS board + trie prune · A tree of prefixes

Trie of reverses / hashmap of reverses · A tree of prefixes

Trie or startsWith · A tree of prefixes

Drop words that are suffixes of others · A tree of prefixes

Flood fill components · Nodes, edges, and a visited set

Component size · Nodes, edges, and a visited set

Map old node → copy · Nodes, edges, and a visited set

Multi-source BFS minutes · Nodes, edges, and a visited set

Cycle detection / Kahn · Nodes, edges, and a visited set

Topological order · Nodes, edges, and a visited set

Flow uphill from oceans · Nodes, edges, and a visited set

UF or DFS count · Nodes, edges, and a visited set

N-1 edges and connected · Nodes, edges, and a visited set

Edge that creates a cycle · Nodes, edges, and a visited set

Recolor a 4-connected component · Nodes, edges, and a visited set

Visit with collected keys · Nodes, edges, and a visited set

Dijkstra from k · Grow a minimum spanning tree with a heap

Bellman-Ford k+1 rounds · Grow a minimum spanning tree with a heap

Prim / Kruskal Manhattan · Grow a minimum spanning tree with a heap

Min-max path · Grow a minimum spanning tree with a heap

Letter graph + topo · Grow a minimum spanning tree with a heap

Hierholzer lexical Euler path · Grow a minimum spanning tree with a heap

Binary search + BFS / Dijkstra · Grow a minimum spanning tree with a heap

Union emails, then sort · Grow a minimum spanning tree with a heap

Tarjan bridges · Grow a minimum spanning tree with a heap

Dijkstra on -log probability · Grow a minimum spanning tree with a heap

All-pairs shortest paths then count · Grow a minimum spanning tree with a heap

Count shortest paths · Grow a minimum spanning tree with a heap

Sort then merge overlap · Sort intervals to merge overlaps

Splice then merge · Sort intervals to merge overlaps

Min rooms via heap of ends · Sort intervals to merge overlaps

Keep earliest end · Sort intervals to merge overlaps

Kadane · Sort intervals to merge overlaps

Farthest reachable · Sort intervals to merge overlaps

Jumps by range expansion · Sort intervals to merge overlaps

Unique start if total gas >= cost · Sort intervals to merge overlaps

Extend to last occurrence · Sort intervals to merge overlaps

Greedy groups of groupSize · Sort intervals to merge overlaps

Plant in a 0 with 0 neighbors · Sort intervals to merge overlaps

Smallest cookie that satisfies · Sort intervals to merge overlaps

Bottom-up Fibonacci · One index, a recurrence, a table

Dp[i] = cost[i] + min(dp[i+1], dp[i+2]) · One index, a recurrence, a table

Rob or skip adjacent · One index, a recurrence, a table

Linear robber on two ranges · One index, a recurrence, a table

Fewest coins to amount · One index, a recurrence, a table

1-digit / 2-digit splits · One index, a recurrence, a table

Track min and max products · One index, a recurrence, a table

Dp prefix in dictionary · One index, a recurrence, a table

Patience sorting / dp · One index, a recurrence, a table

Subset sum to total/2 · One index, a recurrence, a table

T_n = T_{n-1}+T_{n-2}+T_{n-3} · One index, a recurrence, a table

Bucket counts, then rob-or-skip · One index, a recurrence, a table

Right/down combinations · Two indexes: fill grids from their neighbors

Zero out obstacle cells · Two indexes: fill grids from their neighbors

Min from up or left · Two indexes: fill grids from their neighbors

LCS table · Two indexes: fill grids from their neighbors

Insert/delete/replace DP · Two indexes: fill grids from their neighbors

Unbounded knapsack combinations · Two indexes: fill grids from their neighbors

Partition to (total+target)/2 · Two indexes: fill grids from their neighbors

Dp[i][j] = min of three + 1 · Two indexes: fill grids from their neighbors

S3 formed by merging s1 and s2 · Two indexes: fill grids from their neighbors

Ways s can form t · Two indexes: fill grids from their neighbors

Bottom-up adjacent sums · Two indexes: fill grids from their neighbors

Count palindromic slices · Two indexes: fill grids from their neighbors

XOR cancels pairs · XOR tricks, bit counts, and masking

N &= n-1 · XOR tricks, bit counts, and masking

Dp[i] = dp[i>>1] + (i&1) · XOR tricks, bit counts, and masking

XOR 0..n with values · XOR tricks, bit counts, and masking

Build reversed 32-bit · XOR tricks, bit counts, and masking

XOR + AND carry · XOR tricks, bit counts, and masking

Bits that appear 3k+1 times · XOR tricks, bit counts, and masking

Popcount of XOR · XOR tricks, bit counts, and masking

N > 0 && (n & (n-1)) == 0 · XOR tricks, bit counts, and masking

Shift to common prefix · XOR tricks, bit counts, and masking

Flip bits up to the highest 1 · XOR tricks, bit counts, and masking

Xor all, split on a differing bit · XOR tricks, bit counts, and masking

Digit array + 1 · Number theory, geometry, and matrix walks

Floyd on digit-square sums · Number theory, geometry, and matrix walks

Compare reversed digits · Number theory, geometry, and matrix walks

Subtract if smaller than next · Number theory, geometry, and matrix walks

Transpose then reverse rows · Number theory, geometry, and matrix walks

Walk layers clockwise · Number theory, geometry, and matrix walks

Use first row/col as flags · Number theory, geometry, and matrix walks

Sieve of Eratosthenes · Number theory, geometry, and matrix walks

Base-26 decode · Number theory, geometry, and matrix walks

Digit products into an array · Number theory, geometry, and matrix walks

Add from the right with carry · Number theory, geometry, and matrix walks

Integer square root · Number theory, geometry, and matrix walks