【栈】155.最小栈

题目

法1：两个栈实现最小栈

Python

class MinStack:

    def __init__(self):
        self.stack1 = list()
        self.stack2 = list() # min

    def push(self, val: int) -> None:
        self.stack1.append(val)
        if len(self.stack2) == 0 or self.stack2[-1] >= val:
            self.stack2.append(val)

    def pop(self) -> None:
        top_val = self.stack1.pop()
        if top_val == self.stack2[-1]:
            self.stack2.pop()

    def top(self) -> int:
        return self.stack1[-1]

    def getMin(self) -> int:
        return self.stack2[-1]


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

Java

class MinStack {
    Stack<Integer> stack1;
    Stack<Integer> stack2;

    public MinStack() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }
    
    public void push(int val) {
        stack1.push(val);
        if (stack2.isEmpty()) {
            stack2.push(val);
        } else if (val <= stack2.peek()) {
            stack2.push(val);
        }
    }
    
    public void pop() {
        int val = stack1.pop();
        if (val == stack2.peek()) { // 注意：这里需要加"="，会有多个最小元素push进来
            stack2.pop();
        }
    }
    
    public int top() {
        return stack1.peek();
    }
    
    public int getMin() {
        return stack2.peek();
    }
}