Implement Queue using Stacks
2015-08-12 18:52
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Description:
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).
Solution:
class Queue { private: stack<int> input; stack<int> output; public: // Push element x to the back of queue. void push(int x) { input.push(x); } // Removes the element from in front of queue. void pop(void) { peek(); output.pop(); } // Get the front element. int peek(void) { if (output.empty()) { while (!input.empty()) { output.push(input.top()); input.pop(); } } return output.top(); } // Return whether the queue is empty. bool empty(void) { return input.empty() && output.empty(); } };
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