Leetcode 第225题 Implement Stack using Queues
2015-11-27 23:33
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题目:Implement Stack using Queues
Implement the following operations of a stack using queues.push(x) – Push element x onto stack.
pop() – Removes the element on top of the stack.
top() – Get the top element.
empty() – Return whether the stack is empty.
Notes:
You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
题目分析:
题目含义是用队列来模拟栈,包含栈的判断空、入栈、出栈、取出栈顶元素等思路:
定义两个队列a和b判断空:如果队列a和队列b都是空,那么栈也是空
入栈:如果队列a为空,将入栈的元素入队到队列a中,然后将非空的队列b中的元素入队到队列a中;反之如果队列b为空同理
出栈:如果队列a不为空,那么出栈的元素就是队列a中队尾元素;反之对队列b也是如此
返回栈顶元素:取得非空队列中队尾的元素
代码:
C++:class Stack { public: queue<int> a;//队列a queue<int> b;//队列b // Push element x onto stack. void push(int x) { if(a.empty())//如果队列a是空队列,那么将元素x入队到队列a中 { a.push(x); while(!b.empty())//如果队列b为非空队列时,将队列b中的元素入队到队列a中 { int x=b.front(); b.pop(); a.push(x); } } else//如果队列a不是空队列,那么将元素x入队到队列b中 { b.push(x); while(!a.empty())//如果队列a为非空队列时,将队列a中的元素入队到队列b中 { int x=a.front(); a.pop(); b.push(x); } } } // Removes the element on top of the stack. void pop() { if(!a.empty())//如果队列a不是空队列,那么出栈操作操作就是将队列a中的队头元素出队 { a.pop(); } if(!b.empty())//如果队列b不是空队列,那么出栈操作操作就是将队列b中的队头元素出队 { b.pop(); } } // Get the top element. int top() { if(!a.empty())//如果队列a不是空队列,那么获取栈顶元素操作操作就是将队列a中的队头元素取出 { return a.front(); } if(!b.empty())//如果队列b不是空队列,那么获取栈顶元素操作操作就是将队列b中的队头元素取出 { return b.front(); } } // Return whether the stack is empty. bool empty() { return a.empty()&&b.empty(); } };
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