tag: 栈（stack）

题目描述

设计一个支持 push，pop，top 操作，并能在常数时间内检索到最小元素的栈。

push(x) -- 将元素 x 推入栈中。
pop() -- 删除栈顶的元素。
top() -- 获取栈顶元素。
getMin() -- 检索栈中的最小元素。

示例:

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

python版本：

class MinStack(object):

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.stack = []
        self.min = None


    def push(self, x):
        """
        :type x: int
        :rtype: None
        """
        self.stack.append(x)
        if self.min == None or x < self.min:
            self.min = x
        

    def pop(self):
        """
        :rtype: None
        """
        popItem = self.stack.pop()
        if len(self.stack) == 0:
            self.min = None
            return popItem
        
        if popItem == self.min:
            self.min = self.stack[0]
            for i in self.stack:
                if i < self.min:
                    self.min = i
        return popItem

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

        
    def getMin(self):
        """
        :rtype: int
        """           
        return self.min
        

if __name__ == "__main__":
    obj = MinStack()
    obj.push(12)
    obj.push(14)
    obj.push(54)
    print(obj.pop())
    print(obj.top())
    print(obj.getMin())

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

java版本：

import java.util.Stack;

class MinStack {

    /** initialize your data structure here. */
    Stack<Integer> stack = new Stack<Integer>();
    Stack<Integer> min = new Stack<Integer>();

    
    public void push(int x) {
        if (this.stack.isEmpty()){
            this.stack.push(x);
            this.min.push(x);
        } else {
            this.stack.push(x);
            this.min.push(Math.min(this.stack.peek(), this.min.peek()));
        }
    }
    
    public void pop() {
        if (!this.stack.isEmpty()){
            this.stack.pop();
            this.min.pop();
        }
    }
    
    public int top() {
        if (!this.stack.isEmpty()){
            return this.stack.peek();
        }
        throw new IllegalStateException();
    }
    
    public int getMin() {
        if (!this.min.isEmpty()){
            return this.min.peek();
        }
        throw new IllegalStateException();
    }
}

/**
 * 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();
 */