Implementing Stacks 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 the only standard operations of a queue – which means only push to back ,, peek/pop from front size , and is empty operations AR E valid.
• Depending on your language, queue May is not supported natively. You could simulate a queue by using a list or deque (double-ended queue), as long as if you have standard operations of a Q Ueue.
• You may assume this all operations is valid (for example, no pop or top operations would 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.

analyse:using queues to store the information. The used to mock the stack and the used to store temporary values.

1. Time Exceeded Limit Version

It ' s because transfering temporary queue to the original queue costs a lot of time.

1 classStack {2 Private:3queue<int>Q;4queue<int>Help ;5  Public:6     //Push element x onto stack.7     voidPushintx) {8 Q.push (x);9     }Ten  One     //removes the element on top of the stack. A     voidpop () { -          while(Q.size () >1){ - Help.push (Q.front ()); the Q.pop (); -         } - Q.pop (); -          while(!Help.empty ()) { + Q.push (Help.front ()); -         } +     } A  at     //Get the top element. -     intTop () { -          while(Q.size () >1){ - Help.push (Q.front ()); - Q.pop (); -         } in         intresult =Q.front (); -          while(!Help.empty ()) { to Q.push (Help.front ()); +         } -     } the  *     //Return Whether the stack is empty. $     BOOLempty () {Panax Notoginseng         returnq.empty (); -     } the};

View Code

2. Set a 2-size array to respectively represent the When it comes to copy the temporary queue to the original queue, just switch the index.

Runtime:0ms.

1 classStack {2 Private:3queue<int> q[2];4     intindex =0;5  Public:6     //Push element x onto stack.7     voidPushintx) {8 Q[index].push (x);9     }Ten  One     //removes the element on top of the stack. A     voidpop () { -          while(Q[index].size () >1){//Transfer all and elements into the helper queue -q[1-Index].push (Q[index].front ()); the Q[index].pop (); -         } - Q[index].pop (); -index =1-index; +     } -  +     //Get the top element. A     intTop () { at          while(Q[index].size () >1){//Transfer all and elements into the helper queue -q[1-Index].push (Q[index].front ()); - Q[index].pop (); -         } -         intresult =Q[index].front (); -q[1-Index].push (result); in Q[index].pop (); -index =1-index; to         returnresult; +     } -  the     //Return Whether the stack is empty. *     BOOLempty () { $         returnq[index].empty ();Panax Notoginseng     } -};

Implementing Stacks Using Queues