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MediumStacks Queues

Min Stack

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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.

You must implement a solution with O(1) time complexity for each function.

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

Example 2:

Input:
["MinStack","push","push","getMin","pop","getMin"]
[[],[1],[0],[],[],[]]
Output: [null,null,null,0,null,1]
Explanation:
MinStack minStack = new MinStack();
minStack.push(1);
minStack.push(0);
minStack.getMin(); // return 0
minStack.pop();
minStack.getMin(); // return 1

Examples

Example 1

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

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

Explanation: After pushing -2, 0, -3: getMin returns -3. After popping -3: top returns 0 and getMin returns -2.

Example 2

Input: ["MinStack","push","push","getMin","pop","getMin"], [[],[1],[0],[],[],[]]

Output: [null,null,null,0,null,1]

Explanation: After pushing 1 and 0: getMin returns 0. After popping 0: getMin returns 1.

Example 3

Input: ["MinStack","push","push","push","getMin","pop","getMin","pop","getMin"], [[],[2],[1],[3],[],[],[],[],[]]

Output: [null,null,null,null,1,null,1,null,2]

Explanation: After pushing 2, 1, 3: getMin returns 1. After popping 3: getMin still returns 1. After popping 1: getMin returns 2.

Constraints

  • --2^31 <= val <= 2^31 - 1
  • -Methods pop, top and getMin operations will always be called on non-empty stacks.
  • -At most 3 * 10^4 calls will be made to push, pop, top, and getMin.

Optimal Complexity

Time

O(1)

Space

O(n)

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