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Question

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

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• getMin() – Retrieve the minimum element in the stack.

Example:

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

Solution

Result: Accepted Time: 28 ms

Here should be some explanations.

class MinStack {
    stack<int> sta;
    stack<int> min_sta;
public:
    void push(int x) {
        sta.push(x);
        if(min_sta.empty())
        {
            min_sta.push(x);
            return;
        }
        min_sta.push(min(min_sta.top(),x));
    }

    void pop() {
        sta.pop();
        min_sta.pop();
    }

    int top() {
        return sta.top();
    }

    int getMin() {
        return min_sta.top();
    }
};

Complexity Analytics

• Time Complexity: $O(1)$
• Space Complexity: $O(1)$