155. Min Stack 

Description

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.

Solutions

Solution 1

Python3

class MinStack:
    def __init__(self):
        self.stk1 = []
        self.stk2 = [inf]

    def push(self, val: int) -> None:
        self.stk1.append(val)
        self.stk2.append(min(val, self.stk2[-1]))

    def pop(self) -> None:
        self.stk1.pop()
        self.stk2.pop()

    def top(self) -> int:
        return self.stk1[-1]

    def getMin(self) -> int:
        return self.stk2[-1]


# Your MinStack object will be instantiated and called as such:
# obj = MinStack()
# obj.push(val)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.getMin()

Java

class MinStack {
    private Deque<Integer> stk1 = new ArrayDeque<>();
    private Deque<Integer> stk2 = new ArrayDeque<>();

    public MinStack() {
        stk2.push(Integer.MAX_VALUE);
    }

    public void push(int val) {
        stk1.push(val);
        stk2.push(Math.min(val, stk2.peek()));
    }

    public void pop() {
        stk1.pop();
        stk2.pop();
    }

    public int top() {
        return stk1.peek();
    }

    public int getMin() {
        return stk2.peek();
    }
}

/**
 * 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();
 */

C++

class MinStack {
public:
    MinStack() {
        stk2.push(INT_MAX);
    }

    void push(int val) {
        stk1.push(val);
        stk2.push(min(val, stk2.top()));
    }

    void pop() {
        stk1.pop();
        stk2.pop();
    }

    int top() {
        return stk1.top();
    }

    int getMin() {
        return stk2.top();
    }

private:
    stack<int> stk1;
    stack<int> stk2;
};

/**
 * 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();
 */

Go

type MinStack struct {
	stk1 []int
	stk2 []int
}

func Constructor() MinStack {
	return MinStack{[]int{}, []int{math.MaxInt32}}
}

func (this *MinStack) Push(val int) {
	this.stk1 = append(this.stk1, val)
	this.stk2 = append(this.stk2, min(val, this.stk2[len(this.stk2)-1]))
}

func (this *MinStack) Pop() {
	this.stk1 = this.stk1[:len(this.stk1)-1]
	this.stk2 = this.stk2[:len(this.stk2)-1]
}

func (this *MinStack) Top() int {
	return this.stk1[len(this.stk1)-1]
}

func (this *MinStack) GetMin() int {
	return this.stk2[len(this.stk2)-1]
}

/**
 * Your MinStack object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Push(val);
 * obj.Pop();
 * param_3 := obj.Top();
 * param_4 := obj.GetMin();
 */

TypeScript

class MinStack {
    stk1: number[];
    stk2: number[];

    constructor() {
        this.stk1 = [];
        this.stk2 = [Infinity];
    }

    push(val: number): void {
        this.stk1.push(val);
        this.stk2.push(Math.min(val, this.stk2[this.stk2.length - 1]));
    }

    pop(): void {
        this.stk1.pop();
        this.stk2.pop();
    }

    top(): number {
        return this.stk1[this.stk1.length - 1];
    }

    getMin(): number {
        return this.stk2[this.stk2.length - 1];
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * var obj = new MinStack()
 * obj.push(x)
 * obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.getMin()
 */

Rust

use std::collections::VecDeque;
struct MinStack {
    stk1: VecDeque<i32>,
    stk2: VecDeque<i32>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl MinStack {
    fn new() -> Self {
        Self {
            stk1: VecDeque::new(),
            stk2: VecDeque::new(),
        }
    }

    fn push(&mut self, x: i32) {
        self.stk1.push_back(x);
        if self.stk2.is_empty() || *self.stk2.back().unwrap() >= x {
            self.stk2.push_back(x);
        }
    }

    fn pop(&mut self) {
        let val = self.stk1.pop_back().unwrap();
        if *self.stk2.back().unwrap() == val {
            self.stk2.pop_back();
        }
    }

    fn top(&self) -> i32 {
        *self.stk1.back().unwrap()
    }

    fn get_min(&self) -> i32 {
        *self.stk2.back().unwrap()
    }
}

JavaScript

var MinStack = function () {
    this.stk1 = [];
    this.stk2 = [Infinity];
};

/**
 * @param {number} val
 * @return {void}
 */
MinStack.prototype.push = function (val) {
    this.stk1.push(val);
    this.stk2.push(Math.min(this.stk2[this.stk2.length - 1], val));
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
    this.stk1.pop();
    this.stk2.pop();
};

/**
 * @return {number}
 */
MinStack.prototype.top = function () {
    return this.stk1[this.stk1.length - 1];
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () {
    return this.stk2[this.stk2.length - 1];
};

/**
 * Your MinStack object will be instantiated and called as such:
 * var obj = new MinStack()
 * obj.push(val)
 * obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.getMin()
 */

C#

public class MinStack {
    private Stack<int> stk1 = new Stack<int>();
    private Stack<int> stk2 = new Stack<int>();

    public MinStack() {
        stk2.Push(int.MaxValue);
    }

    public void Push(int x) {
        stk1.Push(x);
        stk2.Push(Math.Min(x, GetMin()));
    }

    public void Pop() {
        stk1.Pop();
        stk2.Pop();
    }

    public int Top() {
        return stk1.Peek();
    }

    public int GetMin() {
        return stk2.Peek();
    }
}

/**
 * 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();
 */