155. 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 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

Constraints:

• Methods pop, top and getMin operations will always be called on non-empty stacks.

class MinStack {
public:
    /** initialize your data structure here. */
    stack<int> stk;
    stack<int> minstk;
    MinStack() {

    }

    void push(int x) {
        stk.push(x);
        if(minstk.size() == 0 || x <= minstk.top()){
            minstk.push(x);
        }
    }

    void pop() {
        if(stk.size() == 0) return;
        int x = stk.top();
        stk.pop();
        if(x == minstk.top()){
            minstk.pop();
        }
    }

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

    int getMin() {
        return minstk.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();
 */