155. Min Stack

LeetCode 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(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:

• -2^31 <= val <= 2^31 - 1

• Methods pop, top and getMin operations will always be called on non-empty stacks.

• At most 3 * 10^4 calls will be made to push, pop, top, and getMin.

Tags

Stack, Design

Solution

Apart from a standard stack data structure (an array implemented stack), we also initialize a minStack to keep track of the minimum value of the stack. Operations related to minStack:

• Constructor: minStack is initialized with an element intMax to avoid to be empty when we push the first element into the stack;

• Push: push the smaller element between x and the top of minStack;

• Pop: pop the top of minStack;

• GetMin: return the top of minStack.

Complexity

• Time complexity: $O(1)$

• Space complexity: $O(n)$

Code

type MinStack struct {
	stack, minStack []int
}

func Constructor() MinStack {
	return MinStack{[]int{}, []int{9223372036854775807}}
}

func (this *MinStack) Push(x int) {
	this.stack = append(this.stack, x)
	this.minStack = append(this.minStack, min(x, this.minStack[len(this.minStack)-1]))
}

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

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

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

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

Reference

1. 最小栈