LeetCode 232. Implement Queue using Stacks

232. Implement Queue using Stacks

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Total Accepted: 39637 Total Submissions: 116826 Difficulty: Easy

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;
}
}
};