155. Min Stack

Medium

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.

解題

type MinStack struct {
    min []int
    stack []int
}


func Constructor() MinStack {
    return MinStack{ }
}


func (this *MinStack) Push(val int)  {
    if len(this.stack) == 0 {
        this.stack = []int{ val }
        this.min = []int{ val }
    } else {
        this.stack = append(this.stack, val)

        if val < this.min[len(this.min) - 1]{
            this.min = append(this.min, val)
        } else {
            this.min = append(this.min, this.min[len(this.min) - 1])
        }
    }
}


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


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


func (this *MinStack) GetMin() int {
    return this.min[len(this.min) - 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();
 */