155.Min Stack

Tags: Easy Stack Design

Links: https://leetcode.com/problems/min-stack/

---

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.

---

class MinStack {
    stack<int> s1, s2;
public:
    /** initialize your data structure here. */
    MinStack() {}

    void push(int x) {
        s1.push(x);
        if (s2.empty() || x <= s2.top()) s2.push(x);
    }

    void pop() {
        if (s1.empty()) return;
        if (s1.top() == s2.top()) s2.pop();
        s1.pop();
    }

    int top() {
        if (s1.empty()) return 0;

        return s1.top();
    }

    int getMin() {
        if (s2.empty()) return 0;

        return s2.top();
    }
};

/**
 * 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();
 */