栈、队列和堆

1、用栈实现队列

class MyQueue {
    stack<int> stack1,stack2;
public:

    /** Initialize your data structure here. */
    MyQueue() {
        while(!stack1.empty())
            stack1.pop();
        while(!stack2.empty())
            stack2.pop();
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        stack1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if (stack2.empty())
        {
            while(!stack1.empty())
            {
                int val = stack1.top();
                stack1.pop();
                stack2.push(val);
            }
        }
        if(stack2.empty())
            return -1;
        else
        {
            int val = stack2.top();
            stack2.pop();
            return val;
        }
    }
    
    /** Get the front element. */
    int peek() {
        if (stack2.empty())
        {
            while(!stack1.empty())
            {
                int val = stack1.top();
                stack1.pop();
                stack2.push(val);
            }
        }
        if(stack2.empty())
            return -1;
        return stack2.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return stack1.empty() && stack2.empty();
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */

 

2、用队列实现栈

class MyStack {
    queue<int> queue1; //主队列
    queue<int> queue2; //辅助队列
public:
    /** Initialize your data structure here. */
    MyStack() {
        while(!queue1.empty())
            queue1.pop();
        while(!queue2.empty())
            queue2.pop();
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        queue1.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if (queue1.empty())
            return -1;
        if(queue2.empty())
        {
            while(queue1.size()!=1)
            {
                int val = queue1.front();
                queue1.pop();
                queue2.push(val);
            }    
        }
        int temp = queue1.front();
        queue1.pop();

        while(!queue2.empty())
        {
            int val = queue2.front();
            queue2.pop();
            queue1.push(val);
        }
        return temp;
    }
    
    /** Get the top element. */
    int top() {
        return queue1.back();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return queue1.empty();
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */

 

 

3、最小栈

方法：使用辅助栈

class MinStack {
    stack<int>data_stack;
    stack<int>min_stack;
public:
    /** initialize your data structure here. */
    MinStack() {
        while(!data_stack.empty())
            data_stack.pop();
        while(!min_stack.empty())
            min_stack.pop();
    }
    
    void push(int x) {
        data_stack.push(x);
        if (min_stack.empty())
            min_stack.push(x);
        else{
            if (min_stack.top() > x)
            {
                min_stack.push(x);
            }
            else
                min_stack.push(min_stack.top());
        }
    }
    
    void pop() {
        if (data_stack.empty())
            return;
        data_stack.pop();
        min_stack.pop();
    }
    
    int top() {
        return data_stack.top();
    }
    
    int getMin() {
        return min_stack.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();
 */

 

 

4、验证栈序列

方法：考察堆栈，那么就用堆栈模拟

　　先按 pushed 数组入栈，当入栈元素等于 popped 数组对应的数字之后，把该元素出栈，最后如果正确的话栈肯定是空的

class Solution {
public:
    bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
        stack<int> s;
        int j=0;//popped的index
        for(int i=0;i<pushed.size();++i)
        {
            s.push(pushed[i]);
            while(!s.empty()&&s.top()==popped[j])
            {
                s.pop();
                j++;
            }
        }
        
        while(!s.empty()&&j<popped.size())
        {
            if(s.top()==popped[j])
            {
                s.pop();
                j++;
            }
            else
            {
                return false;
            }
        }
        return true;
    }
};