LeetCode-in-All

155. Min Stack

Easy

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

Example 1:

Input

["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output: [null,null,null,null,-3,null,0,-2]

Explanation:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top(); // return 0
minStack.getMin(); // return -2 

Constraints:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

Solution

-spec min_stack_init_() -> any().
min_stack_init_() ->
    catch ets:delete(min_stack),
    ets:new(min_stack, [named_table, ordered_set, public]),
    ets:insert(min_stack, {size, 0}),
    ok.

-spec min_stack_push(Val :: integer()) -> any().
min_stack_push(Val) ->
    [{size, Size}] = ets:lookup(min_stack, size),
    CurrentMin = case Size of
        0 -> Val;
        _ -> min(Val, min_stack_get_min())
    end,
    ets:insert(min_stack, {Size + 1, {Val, CurrentMin}}),
    ets:insert(min_stack, {size, Size + 1}),
    ok.

-spec min_stack_pop() -> any().
min_stack_pop() ->
    [{size, Size}] = ets:lookup(min_stack, size),
    case Size > 0 of
        true ->
            ets:delete(min_stack, Size),
            ets:insert(min_stack, {size, Size - 1}),
            ok;
        false ->
            ok
    end.

-spec min_stack_top() -> integer().
min_stack_top() ->
    [{size, Size}] = ets:lookup(min_stack, size),
    case Size > 0 of
        true ->
            [{_, {Val, _}}] = ets:lookup(min_stack, Size),
            Val;
        false ->
            0  % Default value for empty stack
    end.

-spec min_stack_get_min() -> integer().
min_stack_get_min() ->
    [{size, Size}] = ets:lookup(min_stack, size),
    case Size > 0 of
        true ->
            [{_, {_, Min}}] = ets:lookup(min_stack, Size),
            Min;
        false ->
            0  % Default value for empty stack
    end.

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