【劍指Offer系列30】包含min函數的棧

題目

定義棧的數據結構，請在該類型中實現一個能夠得到棧的最小元素的 min 函數在該棧中，調用 min、push 及 pop 的時間複雜度都是 O(1)。

代碼

Python

class MinStack:

    def __init__(self):
        """
        initialize your data structure here.
        """
        # A數據棧，B輔助存儲最小值棧
        self.A, self.B = [], []

    def push(self, x: int) -> None:
        self.A.append(x)
        # B爲空/B棧頂元素大於等於壓入元素
        if not self.B or self.B[-1]>=x:
            self.B.append(x)

    def pop(self) -> None:
        # 彈出元素恰好是最小值
        if self.A.pop()==self.B[-1]:
            self.B.pop()

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

    def min(self) -> int:
        return self.B[-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.min()

C++

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {
        
    }
    
    // A數據棧，B輔助棧
    stack<int> A;
    stack<int> B;
    
    void push(int x) {
        A.push(x);
        // 注意兩個條件分開寫，保持出棧一致性
        if (B.empty()) {
            B.push(x);
        }
        else if (B.top() >= x) {
            B.push(x);
        }
    }
    
    void pop() {
        if (!A.empty()) {
            if (A.top()==B.top()) B.pop();
            A.pop();
        }
    }
    
    int top() {
        return A.top();
    }
    
    int min() {
        return B.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->min();
 */