Implement the following operations of a queue using stacks.
push(x) — Push element x to the back of queue.
pop() — Removes the element from in front of queue.
peek() — Get the front element.
empty() — Return whether the queue is empty.
Notes:
You must use only standard operations of a stack — which means only push to top, peek/pop from top, size, and is empty operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
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//用两个 stack 实现队列的操作 //压入s1的元素是倒序的，把它们逐个弹出再压入s2就是正序的了 class Queue { public: stack<int> s1, s2; // Push element x to the back of queue. void push(int x) { s1.push(x); } // Removes the element from in front of queue. void pop(void) { if(s2.empty()) { while(!s1.empty()) { s2.push(s1.top()); s1.pop(); } } s2.pop(); } // Get the front element. int peek(void) { if(s2.empty()) { while(!s1.empty()) { s2.push(s1.top()); s1.pop(); } } return s2.top(); } // Return whether the queue is empty. bool empty(void) { if(s1.empty() && s2.empty()) { return true; } else { return false; } } }; |
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