【栈】剑指offer——面试题7：用两个栈实现队列

力扣

Python

class CQueue:

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

    def appendTail(self, value: int) -> None:
        self.stack1.append(value)

    def deleteHead(self) -> int:
        if len(self.stack2) > 0:
            return self.stack2.pop()
        if len(self.stack1) == 0:
            return -1
        
        while self.stack1:
            tmp = self.stack1.pop()
            self.stack2.append(tmp)
        return self.stack2.pop()


# Your CQueue object will be instantiated and called as such:
# obj = CQueue()
# obj.appendTail(value)
# param_2 = obj.deleteHead()

Java

// 版本1：优化版1
class CQueue {
    public Stack<Integer> s1;
    public Stack<Integer> s2;

    public CQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }
    
    public void appendTail(int value) {
        s1.push(value);
    }
    
    public int deleteHead() {
        if (!s2.isEmpty()) {
            return s2.pop();
        }
        if (s1.size() == 0) {
            return -1;
        }
        while (s1.size() > 0) {
            int val = s1.pop();
            s2.push(val);
        }

        return s2.pop();
    }
}

// 版本2：代码优化之
class CQueue {
    public Stack<Integer> s1;
    public Stack<Integer> s2;

    public CQueue() {
        s1 = new Stack<>();
        s2 = new Stack<>();
    }
    
    public void appendTail(int value) {
        s1.push(value);
    }
    
    public int deleteHead() {
        if (s2.size() > 0) {
            return s2.pop();
        }
        if (s1.size() == 0) {
            return -1;
        }
        while (s1.size() > 0) {
            int val = s1.pop();
            s2.push(val);
        }

        return s2.pop();
    }
}

##Solution1:
注意栈的基本操作与vector略有不同~

class Solution
{
public:
    void push(int node) {
        stack1.push(node);//
    }

    int pop() {
        int ele_top=0;
        if(!stack2.empty()){//stack2不为空，就删除stack2的栈顶元素
            ele_top=stack2.top();//top()返回栈顶元素，但不删除
            stack2.pop();//pop()删除栈顶元素，但不返回该值
            return ele_top;
        }
        else{//stack2为空，要把stack1中的元素依次弹出并压入stack2中
            int ele_temp=0;
            while(!stack1.empty()){
                ele_temp=stack1.top();
                stack2.push(ele_temp);
                stack1.pop();
            }
            ele_top=stack2.top();//返回栈顶元素，但不删除
            stack2.pop();//删除栈顶元素，但不返回该值
            return ele_top;
        }
    }

private:
    stack<int> stack1;
    stack<int> stack2;
};

#20180830重做

class Solution {
public:
    void push(int node) { //元素压入stack1中
        stack1.push(node);
        return;
    }

    int pop() {
        if(stack2.empty()) {
            while (!stack1.empty()) {
                stack2.push(stack1.top());
                stack1.pop();
            }
        }
        int temp = stack2.top();
        stack2.pop();
        return temp;
    }

private:
    stack<int> stack1;
    stack<int> stack2;
};