Implement Queue using Stacks
2015-07-20 00:39
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一、题目要求
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).
二、代码实现
// Push element x to the back of queue.
void push(int x) {
st1.push(x);
}
// Removes the element from in front of queue.
void pop(void) {
if(!st2.empty())
{
st2.pop();
}
else
{
while(!st1.empty())
{
st2.push(st1.top());
st1.pop();
}
st2.pop();
}
}
// Get the front element.
int peek(void) {
if(st2.empty())
{
while(!st1.empty())
{
st2.push(st1.top());
st1.pop();
}
}
return st2.top();
}
// Return whether the queue is empty.
bool empty(void) {
if(st1.empty()&&st2.empty())
return true;
return false;
}
private:
stack<int> st1,st2;
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).
二、代码实现
// Push element x to the back of queue.
void push(int x) {
st1.push(x);
}
// Removes the element from in front of queue.
void pop(void) {
if(!st2.empty())
{
st2.pop();
}
else
{
while(!st1.empty())
{
st2.push(st1.top());
st1.pop();
}
st2.pop();
}
}
// Get the front element.
int peek(void) {
if(st2.empty())
{
while(!st1.empty())
{
st2.push(st1.top());
st1.pop();
}
}
return st2.top();
}
// Return whether the queue is empty.
bool empty(void) {
if(st1.empty()&&st2.empty())
return true;
return false;
}
private:
stack<int> st1,st2;
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