Easy
Queue Using Stacks:Pour Only When the Front Is Empty
Inbound takes every push; outbound serves pop and peek. Pour inbound into outbound only when outbound is empty, or you bury the current front.
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14 questions
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- Easy
Stack Using Queues:Rotate Until the Newest Is Front
Offer then rotate size-1 so the newest sits at the front; pop and top become poll and peek, and that push is O(n) every time, not amortized.
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Valid Parentheses:Push Opens, Pop on a Match
A per-type count accepts ([)]. Push every open; a close is legal only if the top matches, and the stack is empty when the string ends.
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- Medium
Asteroid Collision:A Left-Goer Pops Smaller Right-Goers
Rescanning the lane after every blast is quadratic. A deque of survivors lets one left-goer pop every smaller right-goer still on top; same-direction rocks never collide.
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Basic Calculator II:Delay Plus and Minus Until Multiply Binds
The operator you apply is the previous one; multiply and divide collapse the top term while plus and minus wait as signed stack entries.
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Car Fleet:A Catch Is an Arrival That Is Not Later
Sort from the target backward and push a fleet time only when this car would arrive strictly later than the leader ahead; equal time at the destination is still one fleet.
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Daily Temperatures:Wait Is Later Index Minus Earlier Index
A nested forward scan is correct and quadratic. A decreasing stack of day indexes answers each wait in one pass; equal highs do not pop.
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Decode String:Push the Count, Expand on the Close
Matching brackets only checks. Digits accumulate a multi-digit k, [ freezes the outer fragment, and ] expands the inner onto it.
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Generate Parentheses:Open While Unused, Close While Unmatched
Finishing every 2^(2n) bitmask then running a checker is honest; illegal prefixes were already dead at the first extra close.
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Min Stack:Store the Floor at This Height
A second deque tracks the running min; push a duplicate when the same floor arrives again, or the first pop of that value lies about getMin.
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Next Greater Element II:The Same Decreasing Stack, Twice Around
The same decreasing index stack as Daily Temperatures; walk 2n, write the greater value, and never push on the second lap.
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Reverse Polish:Push Numbers, Pop Two on an Operator
A stack of pending numbers is the one-pass fold; the first pop is the right operand, and Java int division already truncates toward zero.
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Simplify Path:Push Directories, Pop on Dot-Dot
A stack of live directory names is the one-pass canonicalizer; splicing until the string settles is quadratic, and replace("..", "") smashes `...`.
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Representation and operations — not the problem set.