leetcode--Implement Queue using Stacks
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题目: Implement Queue using Stacks
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.
- You must use only standard operations of a stack -- which means only
push to top
,peek/pop from top
,size
, andis 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).
分析:push即把元素加入栈底,最后一个输出。把栈为123变为x123,借助stack2先存入321,再存入x,再存入123
class MyQueue { // Push element x to the back of queue. private Stack<Integer> stack = new Stack<Integer>(); public void push(int x) { Stack<Integer> stack2 = new Stack<Integer>(); while(stack.size()!=0){ stack2.push(stack.pop()); } stack.push(x); while(stack2.size()!=0){ stack.push(stack2.pop()); } } // Removes the element from in front of queue. public void pop() { stack.pop(); } // Get the front element. public int peek() { return stack.peek(); } // Return whether the queue is empty. public boolean empty() { return stack.isEmpty(); }}Two:
class MyQueue { Stack<Integer> input = new Stack(); Stack<Integer> output = new Stack(); public void push(int x) { input.push(x); } public void pop() { peek(); output.pop(); } public int peek() { if (output.empty()) while (!input.empty()) output.push(input.pop()); return output.peek(); } public boolean empty() { return input.empty() && output.empty(); }}
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