Implement MinStack class with push, pop, top, and getMin methods #133
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WalkthroughAdds a new MinStack implementation in O) Stack/minStack.cpp with O(1) push, pop, top, and getMin using an encoding technique to track the current minimum. Handles empty-stack cases by returning sentinel INT_MIN for top/getMin and initializes currMin with LONG_MAX. Changes
Sequence Diagram(s)sequenceDiagram autonumber actor Client participant MS as MinStack participant S as Internal stack<long> note over MS: State: currMin Client->>MS: push(val) alt stack empty MS->>S: push(val) MS->>MS: currMin = val else val <= currMin MS->>S: push(2*val - currMin) %% encoded MS->>MS: currMin = val else MS->>S: push(val) end Client->>MS: top() alt S.top() < currMin (encoded) MS-->>Client: currMin else MS-->>Client: S.top() end Client->>MS: getMin() MS-->>Client: currMin Client->>MS: pop() alt S.top() < currMin (encoded) MS->>MS: currMin = 2*currMin - S.top() MS->>S: pop() else MS->>S: pop() end note over MS,S: Empty checks return INT_MIN for top/getMin Estimated code review effort🎯 3 (Moderate) | ⏱️ ~20 minutes Poem
Pre-merge checks and finishing touches✅ Passed checks (3 passed)
✨ Finishing touches
🧪 Generate unit tests (beta)
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Actionable comments posted: 4
🧹 Nitpick comments (1)
O) Stack/minStack.cpp (1)
29-35: Encoding triggers unnecessarily when val equals currMin.When
val == currMin, the code encodes the value even though the minimum doesn't change. While functionally correct, this adds overhead. Consider changing line 29 toif (val >= currMin)to avoid encoding when the value equals the current minimum.Apply this diff to optimize:
- if (val > currMin) + if (val >= currMin) st.push(val); elseNote: This is a minor optimization and the current implementation is correct.
| stack<long> st; | ||
| long currMin; |
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Consider using fixed-width types for guaranteed overflow safety.
The stack stores long values to handle the encoding arithmetic 2*val - currMin, but long is only guaranteed to be at least 32 bits by the C++ standard. On platforms where long is 32 bits (e.g., Windows with MSVC), this encoding can overflow when val approaches INT_MIN or INT_MAX.
Apply this diff to use guaranteed 64-bit storage:
+#include <cstdint> + private: - stack<long> st; - long currMin; + stack<int64_t> st; + int64_t currMin;Then update the casts and operations accordingly (e.g., line 33: 2 * (int64_t)val).
🤖 Prompt for AI Agents
In O) Stack/minStack.cpp around lines 9 to 10, the code uses `long` for the stack and `currMin`, which is not guaranteed 64-bit and can overflow during the `2*val - currMin` encoding; replace `long` with a fixed-width type such as `int64_t` for both the stack element type and `currMin`, add the necessary header (<cstdint> or <stdint.h>), and update all casts and arithmetic to use `int64_t` (e.g., change occurrences like `2 * (int64_t)val` and `2*val - currMin` to use int64_t operands) so encoding/decoding cannot overflow on 32-bit long platforms. | long top = st.top(); | ||
| if (top < currMin) | ||
| currMin = 2 * currMin - top; |
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Risk of integer overflow in minimum restoration.
The expression 2 * currMin - top on line 50 can overflow if long is 32 bits and values are extreme. For example, if currMin is large positive and top is large negative (encoded), the computation can exceed the range of a 32-bit long.
This is mitigated by using int64_t as suggested in the earlier comment.
🤖 Prompt for AI Agents
In O) Stack/minStack.cpp around lines 48 to 50, the expression `2 * currMin - top` can overflow with 32-bit long; change the relevant variables and stack element type to 64-bit integers (e.g., int64_t) and perform the subtraction using int64_t temporaries/casts so the arithmetic is done in 64-bit; update currMin, top (and the stack template type) to int64_t and include <cstdint> if needed to ensure safe, overflow-free restoration of the previous minimum. | int top() | ||
| { | ||
| if (st.empty()) | ||
| return INT_MIN; | ||
| | ||
| long top = st.top(); | ||
| if (top < currMin) | ||
| return currMin; | ||
| else | ||
| return top; | ||
| } |
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Sentinel value INT_MIN creates ambiguity.
Returning INT_MIN for an empty stack (line 61) is problematic because INT_MIN is a valid value that can be pushed onto the stack. A caller cannot distinguish between an empty stack and a stack whose top element is INT_MIN.
Consider these alternatives:
- Throw an exception (e.g.,
std::runtime_error) when the stack is empty - Use
std::optional<int>as the return type - Require callers to check emptiness before calling
top()
Example using std::optional:
#include <optional> std::optional<int> top() { if (st.empty()) return std::nullopt; long top = st.top(); if (top < currMin) return currMin; else return top; }🤖 Prompt for AI Agents
O) Stack/minStack.cpp around lines 58-68: the top() function currently returns INT_MIN on empty which is ambiguous; change the signature to return std::optional<int>, add #include <optional>, return std::nullopt when st.empty(), and otherwise return the computed top value (convert/cast to int as needed); update any callers to handle std::optional (or document that callers must check has_value()) and adjust any forward declarations or interface files to the new return type. | int getMin() | ||
| { | ||
| if (st.empty()) | ||
| return INT_MIN; | ||
| return currMin; | ||
| } |
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Sentinel value INT_MIN creates ambiguity.
Similar to the top() method, returning INT_MIN for an empty stack creates ambiguity. If the stack contains INT_MIN as its minimum, getMin() would return the same value as it would for an empty stack.
Apply the same solution as suggested for top() (exception, std::optional, or documented precondition).
🤖 Prompt for AI Agents
In O) Stack/minStack.cpp around lines 74 to 79, getMin() currently returns INT_MIN for an empty stack which is ambiguous; change the API to avoid sentinel values by returning std::optional<int> (include <optional>), update the signature to std::optional<int> getMin(), return std::nullopt when st.empty(), and return currMin otherwise; alternatively, if you prefer exceptions, change it to throw a std::runtime_error on empty and update callers accordingly—ensure all call sites are updated to handle the new return/exception behavior and add a brief comment documenting the precondition/behavior. 
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