Add stack-queue module: ten stack and queue interview problems, monotonic stack, min stack, queue from stacks and stack from a queue, brute-force cross-checks, dry-run traces, operation counts, pitfalls (11 tests)

Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_017ic4b6nxynun5JFivebVb1
2026-11-12 22:35:43 +05:30
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+Eight classic mistakes, each reproduced
+=======================================
+
+1. balanced brackets by counting each kind, with no stack
+   input "([)]":  counts-only = true   stack = false
+   every count ends at zero, but the ] arrives while the ( is not the innermost open bracket
+
+2. min stack that compares boxed Integers with ==
+   push [10, 20, 5], pop, getMin():  broken = 10   correct = 10
+   push [1000, 2000, 500], pop, getMin():  broken = 500   correct = 1000
+   Integer.valueOf caches -128..127, so small values share one object and == happens to work
+
+3. min stack that pushes onto the min stack only on a strictly smaller value
+   push 3, push 3, pop, getMin():  strict -> NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because the return value of "java.util.Deque.peek()" is null
+   correct -> 3   (the stack still holds one 3)
+
+4. encoded min stack with int differences instead of long
+   push Integer.MIN_VALUE, push Integer.MAX_VALUE, top():  int diffs = -2147483648   long diffs = 2147483647
+   MAX_VALUE - MIN_VALUE does not fit in an int, wraps to -1, and -1 looks like "new minimum"
+
+5. next greater element popping on <= instead of <
+   input [2, 2, 3]:  buggy = [2, 3, -1]   correct = [3, 3, -1]
+   the first 2 is told its next greater element is 2
+
+6. largest rectangle without the final height-0 bar
+   input [2, 4]:  buggy = 0   correct = 4
+   nothing ever pops the bars that are still on the stack when the loop ends
+
+7. queue from two stacks that pours on every dequeue
+   enqueue 1, 2; dequeue; enqueue 3; dequeue:  first = 1 (ok), second: broken = 3   correct = 2
+   the 3 was poured on top of the 2 that was still waiting in the output stack
+
+8. popping an empty structure: both conversions throw NoSuchElementException, as ArrayDeque.pop does