Min Stack

Description

Min Stack - LeetCode

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.

Examples

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

• $-2^{31}<=\text{val}<=2^{31}-1$
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most $3\ast 10^4$ calls will be made to push, pop, top, and getMin

Code

# include <vector>
using namespace std;
 
class MinStack {
public:
    vector<int> stack;
    vector<int> mins;
 
    MinStack() {}
 
    void push(int val) {
        if (mins.size() == 0){
            mins.push_back(val);
        } else if (val <= mins.back()){
            mins.push_back(val);
        }
        stack.push_back(val);
    }
 
    void pop() {
        if (stack.back() == mins.back()) {mins.pop_back();}
        stack.pop_back();
    }
 
    int top() {
        return stack.back();
    }
 
    int getMin() {
        return mins.back();
    }
};

Approach

1. Create 2 member variables for MinStack class
   • stack: the actual stack data structure of the class
   • mins: the stack of the minimal value in the stack
2. push member function
   • Check if the mins stack is empty (kick starting the mins member stack)
   • Check if the input value is less than or equal to the top of the mins stack
     • Push to the top of the mins stack
   • Push value to the normal stack
3. pop member function
   • Check if the top of the normal stack (that is about to be removed) equal to the top of the mins stack
     • Remove the top of the mins stack
   • Remove the top of the mins stack
4. top member function
   • Return the top of the normal stack
5. getMin member function
   • Return the top of the mins stack