155. Min Stack

• Difficulty: Easy

• Topics: Stack, Design

• Similar Questions:

  • Sliding Window Maximum

  • Max Stack

Problem:

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.

Solutions:

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {

    }

    void push(int x) {
        if (mins.empty() || mins.top() >= x) {
            mins.push(x);
        }
        stk.push(x);
    }

    void pop() {
        int num = stk.top(); stk.pop();
        if (!mins.empty() && num == mins.top()) {
            mins.pop();
        } 
    }

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

    int getMin() {
        if (mins.empty()) {
            return -1;
        }
        return mins.top();
    }
private:
    stack<int> stk;
    stack<int> mins;

};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack* obj = new MinStack();
 * obj->push(x);
 * obj->pop();
 * int param_3 = obj->top();
 * int param_4 = obj->getMin();
 */