LeetCode:Implement Stack using Queues

Posted 2020-07-10 walker lee

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;
};