LeetCode-in-Cpp

155. Min Stack

Easy

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.

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

#include <stack>
#include <algorithm>

class MinStack {
private:
    struct Node {
        int min;
        int data;
        Node* nextNode;
        Node* previousNode;

        Node(int min, int data, Node* previousNode, Node* nextNode)
            : min(min), data(data), previousNode(previousNode), nextNode(nextNode) {}
    };

    Node* currentNode;

public:
    MinStack() : currentNode(nullptr) {}

    void push(int val) {
        if (currentNode == nullptr) {
            currentNode = new Node(val, val, nullptr, nullptr);
        } else {
            currentNode->nextNode = new Node(std::min(currentNode->min, val), val, currentNode, nullptr);
            currentNode = currentNode->nextNode;
        }
    }

    void pop() {
        if (currentNode != nullptr) {
            currentNode = currentNode->previousNode;
        }
    }

    int top() {
        if (currentNode != nullptr) {
            return currentNode->data;
        }
        return -1;
    }

    int getMin() {
        if (currentNode != nullptr) {
            return currentNode->min;
        }
        return -1;
    }

    ~MinStack() {
        while (currentNode != nullptr) {
            Node* temp = currentNode;
            currentNode = currentNode->previousNode;
            delete temp;
        }
    }
};

/**
 * 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();
 */

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