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, 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).
用栈来实现队列，对比类似题目225. Implement Stack using Queues 是反过来用。
class Queue:
# initialize your data structure here.
def __init__(self):
self.A, self.B = [], []
# @param x, an integer
# @return nothing
def push(self, x):
self.A.append(x)
# @return an integer
def pop(self):
self.peek()
return self.B.pop()
# @return an integer
def peek(self):
if not self.B:
while self.A:
self.B.append(self.A.pop())
return self.B[-1]
# @return an boolean
def empty(self):
return not self.A and not self.B
C++:
class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
stack<int> tmp;
while (!s.empty()) {
tmp.push(s.top());
s.pop();
}
s.push(x);
while (!tmp.empty()) {
s.push(tmp.top());
tmp.pop();
}
}
// Removes the element from in front of queue.
void pop(void) {
s.pop();
}
// Get the front element.
int peek(void) {
return s.top();
}
// Return whether the queue is empty.
bool empty(void) {
return s.empty();
}
private:
stack<int> s;
};
C++:
class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
_new.push(x);
}
void shiftStack() {
if (_old.empty()) {
while (!_new.empty()) {
_old.push(_new.top());
_new.pop();
}
}
}
// Removes the element from in front of queue.
void pop(void) {
shiftStack();
if (!_old.empty()) _old.pop();
}
// Get the front element.
int peek(void) {
shiftStack();
if (!_old.empty()) return _old.top();
return 0;
}
// Return whether the queue is empty.
bool empty(void) {
return _old.empty() && _new.empty();
}
private:
stack<int> _old, _new;
};
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