LeetCode:Implement Stack using Queues

Implement Stack using Queues

Total Accepted: 39756 Total Submissions: 130128 Difficulty: Easy

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, and is 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).

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.

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 (E) Implement Queue using Stacks

c++ code:

class Stack {public:    // Push element x onto stack.    void push(int x) {        q1.push(x);    }    // Removes the element on top of the stack.    void pop() {        while(q1.size() > 1) {            int tmp = q1.front();            q1.pop();            q2.push(tmp);        }        q1.pop();        while(!q2.empty()) {            int tmp = q2.front();            q2.pop();            q1.push(tmp);        }    }    // Get the top element.    int top() {        while(q1.size() > 1) {            int tmp = q1.front();            q1.pop();            q2.push(tmp);        }        int ret = q1.front();        int t = q1.front();        q1.pop();        q2.push(t);        while(!q2.empty()) {            int tmp = q2.front();            q2.pop();            q1.push(tmp);        }        return ret;    }    // Return whether the stack is empty.    bool empty() {        return q1.empty() && q2.empty();    }private:    queue<int> q1,q2;};