LeetCode-in-All

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

// Node class to represent each element in the stack
class Node {
  int min;
  int data;
  Node? previousNode;
  Node? nextNode;

  Node(this.min, this.data, this.previousNode, this.nextNode);
}

class MinStack {
  Node? currentNode;

  // Initialize your data structure here
  MinStack();

  // Push a value onto the stack
  void push(int val) {
    if (currentNode == null) {
      currentNode = Node(val, val, null, null);
    } else {
      currentNode!.nextNode = Node(
        (currentNode!.min < val) ? currentNode!.min : val,
        val,
        currentNode,
        null,
      );
      currentNode = currentNode!.nextNode;
    }
  }

  // Remove the element on top of the stack
  void pop() {
    if (currentNode != null) {
      currentNode = currentNode!.previousNode;
    }
  }

  // Get the top element
  int top() {
    return currentNode!.data;
  }

  // Retrieve the minimum element in the stack
  int getMin() {
    return currentNode!.min;
  }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = MinStack();
 * obj.push(val);
 * obj.pop();
 * int param3 = obj.top();
 * int param4 = obj.getMin();
 */

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