155. Min Stack

Description

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.

Idea

Use a stack to record the minimum.

Solution

class MinStack {
private:
    stack<int> st;
    stack<int> minimum;
public:
    /** initialize your data structure here. */
    MinStack() {
        minimum.push(INT_MAX);
    }
    
    void push(int x) {
        st.push(x);
        if (x <= minimum.top()) {
            minimum.push(x);
        }
    }
    
    void pop() {
        if (st.empty()) {
            return;
        }
        if (st.top() == minimum.top()) {
            minimum.pop();
        }
        st.pop();
    }
    
    int top() {
        if (st.empty()) {
            return -1;
        }
        return st.top();
    }
    
    int getMin() {
        if (st.empty()) {
            return -1;
        }
        return minimum.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();
 */