LeetCode #155 Min Stack 最小栈

Description:
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 1:

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.

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.

思路:
栈可以用数组实现或者链表实现

1. 用一个栈记录最小值
2. 将最小值直接记录在这个栈
   如, 输入 -2 -> 0 -> -3, 栈中的记录依次为 -2 -> -2 -> 0 -> -2 -> -3 -> -3
   获取最小值直接返回最后一项即可

空间换时间
时间复杂度O(1), 空间复杂度O(n)

代码:
C++:

class MinStack {
public:
    /** initialize your data structure here. */
    stack data;
    stack minstack;
    MinStack() {
    }
    
    void push(int x) {
        data.push(x);
        if (minstack.empty() || x <= minstack.top()) minstack.push(x);
    }
    
    void pop() {
        if (data.top() == minstack.top()) minstack.pop();
        data.pop();
    }
    
    int top() {
        return data.top();
    }
    
    int getMin() {
        return minstack.top();
    }
};

/**
 * 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();
 */

Java:

class MinStack {

    /** initialize your data structure here. */
    private Stack stack;
    public MinStack() {
        stack = new Stack<>();
    }
    
    public void push(int x) {
        if (stack.empty()) {
            stack.push(x);
            stack.push(x);
        } else {
            int temp = stack.peek();
            stack.push(x);
            if (temp < x) stack.push(temp);
            else stack.push(x);
        }
    }
    
    public void pop() {
        stack.pop();
        stack.pop();
    }
    
    public int top() {
        if (stack.size() != 0) return stack.get(stack.size() - 2);
        return 0;
    }
    
    public int getMin() {
        if (stack.size() != 0) return stack.get(stack.size() - 1);
        return 0;
    }
}

/**
 * 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:

class MinStack:

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.data = []

    def push(self, x: int) -> None:
        if not len(self.data):
            self.data.append(x)
            self.data.append(x)
        else:
            temp = self.data[-1]
            self.data.append(x)
            if temp < x:
                self.data.append(temp)
            else:
                self.data.append(x)

    def pop(self) -> None:
        self.data.pop()
        self.data.pop()

    def top(self) -> int:
        if len(self.data):
            return self.data[-2]

    def getMin(self) -> int:
        if len(self.data):
            return self.data[-1]


# 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()