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:

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

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

    }

    void push(int x) {
        s1.push(x);
        if ( min_s.empty() ) min_s.push(x);
        else {
        // note "<=" here, to take care of duplicates
            if ( x <= min_s.top() ) min_s.push(x);
        }
        return;
    }

    void pop() {
        if ( min_s.top() == s1.top() ) min_s.pop();
        s1.pop();
    }

    int top() {
        return s1.top();
    }

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