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232. 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.
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peek() — Get the front element.
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empty() — Return whether the queue is empty.
Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns falseNotes:
-
You must use only standard operations of a stack — which means only
push to top,peek/pop from top,size, andis emptyoperations 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).
思路分析
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/**
* @author D瓜哥 · https://www.diguage.com
* @since 2024-09-15 21:11:15
*/
class MyQueue {
Deque<Integer> inStack;
Deque<Integer> outStack;
public MyQueue() {
inStack = new LinkedList<>();
outStack = new LinkedList<>();
}
public void push(int x) {
inStack.push(x);
}
public int pop() {
int peek = peek();
outStack.pop();
return peek;
}
public int peek() {
if (!outStack.isEmpty()) {
return outStack.peek();
} else {
while (!inStack.isEmpty()) {
outStack.push(inStack.pop());
}
}
// TODO 这里， outStack 有可能是空啊？！
return outStack.peek();
}
public boolean empty() {
return inStack.isEmpty() && outStack.isEmpty();
}
}