leetcode:Implement Stack using Queues 与 Implement Queue using Stacks
一、Implement Stack using Queues
Implement the following operations of a stack using queues.
- push(x) -- Push element x onto stack.
- pop() -- Removes the element on top of the stack.
- top() -- Get the top element.
- empty() -- Return whether the stack is empty.
Notes:
- You must use only standard operations of a queue -- which means only
push to back
,peek/pop from front
,size
, andis empty
operations are valid. - Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
- You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
分析:用(两个)队列实现栈
设置两个队列分别为1和2
(1)入栈:如果队列2当前有元素,而队列1为空(反之亦然),那么将需要入栈的元素放入队列1中,然后将队列2中的元素依次出队并入队到队列1中。(即要保证有一个队列是空的)
(2)出栈:将有元素(不为空)的队列出队即可-------如:
先将元素a插入队列1中 ,现在要将元素b入栈,则将b插入到队列2中然后将队列1中的a出队到队列2中,则队列2中的元素变为 b,a
这样队列1为空,现在要压入c, 则将c插入队列1中 ,依次将队列2中的b ,a出队并加入到队列1中 ,则队列1中的元素变为 c,b,a,而队列2为空
(保证一队列为空)
代码如下:
【两个队列】
class Stack {
public:
// Push element x onto stack.
queue<int> queue1;
queue<int> queue2;
void push(int x) {
if (queue1.empty())
{
queue1.push(x);
while(!queue2.empty()){
int tmp = queue2.front();
queue2.pop();
queue1.push(tmp);
}
}else{
queue2.push(x);
while(!queue1.empty()){
int tmp = queue1.front();
queue1.pop();
queue2.push(tmp);
}
}
} // Removes the element on top of the stack.
void pop() {
if (!queue1.empty())
queue1.pop();
if (!queue2.empty())
queue2.pop();
} // Get the top element.
int top() {
if (!queue1.empty())
return queue1.front();
if (!queue2.empty())
return queue2.front();
} // Return whether the stack is empty.
bool empty() {
return queue1.empty() && queue2.empty();
}
};
其他解法:
【两个队列】用两个队列myStack,temp实现一个栈。push时把新元素添加到myStack的队尾。pop时把myStack中除最后一个元素外逐个添加到myStack中,然后pop掉myStack中的最后一个元素,然后注意记得myStack和temp,以保证我们添加元素时始终向temp中添加。
class Stack {
public:
// Push element x onto stack.
void push(int x) {
myStack.push(x);
} // Removes the element on top of the stack.
void pop() {
std::queue<int> temp;
int len = myStack.size();
for(int i = 0; i < len - 1; i++) {
temp.push(myStack.front());
myStack.pop();
}
myStack = temp;
} // Get the top element.
int top() {
if(myStack.size() != 0) return myStack.back();
} // Return whether the stack is empty.
bool empty() {
if(myStack.size() == 0) return true;
else return false;
}
private:
std::queue<int> myStack;
};
或:
【两个队列】
class Stack {
queue<int> rev_q;
public:
// Push element x onto stack.
void push(int x) {
queue<int> temp_q;
temp_q.push(x);
while (!rev_q.empty()) {
temp_q.push(rev_q.front());
rev_q.pop();
} rev_q = temp_q;
} // Removes the element on top of the stack.
void pop() {
rev_q.pop();
} // Get the top element.
int top() {
return rev_q.front();
} // Return whether the stack is empty.
bool empty() {
return rev_q.empty();
}
};
【一个队列】---push时直接添加到队尾就好。pop和top时,把队列除最后一个元素外,逐个循环添加到队列的尾部。
class Stack {
public:
// Push element x onto stack.
void push(int x) {
unsigned int size = s.size();
this->s.push(x);
while (size--){
s.push(s.front());
s.pop();
}
} // Removes the element on top of the stack.
void pop() {
s.pop();
} // Get the top element.
int top() {
return s.front();
} // Return whether the stack is empty.
bool empty() {
return s.empty();
}
private:
queue<int> s;
};
附注:队列queue的成员函数
- empty()判断队列空,当队列空时,返回true。
- size()访问队列中的元素个数。
- push()会将一个元素置入queue中。
- front()会返回queue内的第一个元素(也就是第一个被置入的元素)。
- back()会返回queue中最后一个元素(也就是最后被插入的元素)。
- pop()会从queue中移除一个元素。[1]
- 注意:pop()虽然会移除下一个元素,但是并不返回它,front()和back()返回下一个元素但并不移除该元素。
二、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.
Notes:
- 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:
class Queue {
public:
stack<int> stack1;
stack<int> stack2;
// Push element x to the back of queue.
void push(int x) {
stack1.push(x);
} // Removes the element from in front of queue.
void pop(void) {
if(!stack2.empty()) stack2.pop();
else{
while(!stack1.empty()){
stack2.push(stack1.top());
stack1.pop();
}
stack2.pop();
}
} // Get the front element.
int peek(void) {
if(!stack2.empty()) return stack2.top();
else{
while(!stack1.empty()){
stack2.push(stack1.top());
stack1.pop();
}
return stack2.top();
}
} // Return whether the queue is empty.
bool empty(void) {
return stack1.empty() && stack2.empty();
}
};
注:stack2.push(stack1.top())中若写成 stack2.push(stack1.pop())则会出错:invalid use of void expression
或:
class Queue {
public:
stack<int> s1;
stack<int> 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(s1.empty())
return;
while(!s1.empty()) {
s2.push(s1.top());
s1.pop();
}
s2.pop();
while(!s2.empty()) {
s1.push(s2.top());
s2.pop();
}
} // Get the front element.
int peek(void) {
if(s1.empty())
return -1;
while(!s1.empty()) {
s2.push(s1.top());
s1.pop();
}
int t = s2.top();
while(!s2.empty()) {
s1.push(s2.top());
s2.pop();
}
return t;
} // Return whether the queue is empty.
bool empty(void) {
return s1.empty();
}
};
可参考其他解法:
我们做过一道相反的题目Implement Stack using Queues 用队列来实现栈。这道题颠倒了个顺序,起始并没有太大的区别,栈和队列的核心不同点就是栈是先进后出,而队列是先进先出,那么怎么用栈的先进后出的特性来表示出队列的先进先出呢?方法是:只要在插入元素的时候每次都从前面插入即可,即如果一个队列是1,2,3,4,那么就在栈中保存为4,3,2,1,那么返回栈顶元素1,即为队列的首元素。我们可以设置一个辅助栈tmp,把s的元素也逆着顺序存入tmp中,此时若加入新元素x,再把tmp中的元素倒回来。
代码如下:
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;
};
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