Min Stack Problem


Description

LeetCode Problem 155.

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:

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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:

  • -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.


Sample C++ Code

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class MinStack {
public:
    stack<int> st;
    stack<int> minst;
    
    MinStack() {
        
    }
    
    void push(int x) {
        st.push(x);
        int y;
        if (!minst.empty()) {
            y = minst.top();
            minst.push(min(x, y));
        } else {
            minst.push(x);
        }
    }
    
    void pop() {
        st.pop();
        minst.pop();
    }
    
    int top() {
        if (st.empty())
            return 0;
        return st.top();
    }
    
    int getMin() {
        if (minst.empty())
            return 0;
        return minst.top();
    }
};




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