卖萌的弱渣

I am stupid, I am hungry.

Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

  • push(x) – Push element x onto stack.
  • pop() – Removes the element on top of the stack.
  • top() – Get the top element.
  • getMin() – Retrieve the minimum element in the stack.

Example

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MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

Solution

  • Java: One Stack
(Min-Stack.java) download
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public class MinStack {
    /** initialize your data structure here. */
    int min;
    Stack<Integer> stack;

    public MinStack() {
        stack = new Stack<Integer>();
        min = Integer.Max_VALUE;
    }
    public void push(int x) {
      // only push the old minimum value when the current 
       // minimum value changes after pushing the new value x
        if (x <= min){
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }
    public void pop() {
       // if pop operation could result in the changing of the current minimum value, 
       // pop twice and change the current minimum value to the last minimum value.
        if(stack.peek() == min){
            stack.pop();
            min = stack.pop();
        }
        else
            stack.pop();
        if(stack.empty())
            min = Integer.MAX_VALUE;

    }
    public int top() {
        return stack.peek();
    }
    public int getMin() {
        return min;
    }
}

/**
 * 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();
 */
  • Python: Two stack
(Min-Stack.py) download
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class MinStack(object):

    def __init__(self):
        # do some intialize if necessary
        self.stack = []
        self.minstack = []

    def push(self, number):
        # write yout code here
        self.stack.append(number)
        if len(self.minstack) == 0 or number <= self.minstack[-1]:
            self.minstack.append(number)

    def pop(self):
        # pop and return the top item in stack
        if self.stack[-1] == self.minstack[-1]:
            self.minstack.pop()
        return self.stack.pop()

    def top(self):
        return self.stack[-1]

    def getMin(self):
        # return the minimum number in stack
        return self.minstack[-1]