Min Stack

Problem Statement

Leetcode: 155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

You must implement a solution with O(1) time complexity for each function.

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:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

Solution

class Node {
public:
    int val, min;
    Node(int v, int m) {
        val = v;
        min = m;
    }
};

class MinStack {
    vector<Node> container;
public:
    MinStack() {
        container = vector<Node>();
    }

    void push(int val) {
        int minVal = val;
        if (container.size()) {
            minVal = min(container.back().min, val);
        }
        container.push_back(Node(val, minVal));
    }
    
    void pop() {
        container.pop_back();
    }
    
    int top() {
        return container.back().val;
    }
    
    int getMin() {
        return container.back().min;
    }
};
/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack* obj = new MinStack();
 * obj->push(val);
 * obj->pop();
 * int param_3 = obj->top();
 * int param_4 = obj->getMin();
 */