[LeetCode] Implement Stack using Queues
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.
You must use only standard operations of a queue -- which means only
push to back,peek/pop from front,size, andis emptyoperations 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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.
Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.
解题思路：
用两个队列来模拟栈。记录当前目标队列。压入栈（push）时，直接将元素压入目标队列。弹出栈（pop）时，将目标队列的元素都放入另外一个队列中，只留下一个元素。然后将这个元素弹出。同pop操作一样，top操作将目标队列的元素都放入另外一个队列中，只留下一个元素，然后将这个元素返回。pop操作和top操作注意目标队列的临界值。若目标队列为空，将缓存队列作为目标队列。代码如下：
class Stack {
public:
// Push element x onto stack.
void push(int x) {
queueArr[currentQueue].push(x);
}
// Removes the element on top of the stack.
void pop() {
if(queueArr[currentQueue].empty()){
currentQueue = (currentQueue + 1)%2;
}
while(queueArr[currentQueue].size()!=1){
queueArr[(currentQueue + 1)%2].push(queueArr[currentQueue].front());
queueArr[currentQueue].pop();
}
queueArr[currentQueue].pop();
}
// Get the top element.
int top() {
if(queueArr[currentQueue].empty()){
currentQueue = (currentQueue + 1)%2;
}
while(queueArr[currentQueue].size()!=1){
queueArr[(currentQueue + 1)%2].push(queueArr[currentQueue].front());
queueArr[currentQueue].pop();
}
return queueArr[currentQueue].front();
}
// Return whether the stack is empty.
bool empty() {
return queueArr[0].empty() && queueArr[1].empty();
}
Stack(){
currentQueue = 0;
}
private:
queue<int> queueArr[2];
int currentQueue;
};
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