Leetcode: Implement Stack using Queues

Implement Stack using Queues

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Total Accepted: 28206 Total Submissions: 92981 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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 Stack Design

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------------------------------------------------------------------------

stack.pop() using queue.pop():

using queue to push its front values to back until the original back one! Then pop the current front one.


////////////////////////////////////////////////////////////////////////////
class Stack {
public:
    queue<int> que;
    // Push element x onto stack.
    void push(int x) {
        que.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        //using queue itself to push all front values to back until the last one
        for (int i=0;i<que.size()-1;i++){
            que.push(que.front());
            que.pop();
        }
        que.pop();
    }

    // Get the top element.
    int top() {
        return que.back();
    }

    // Return whether the stack is empty.
    bool empty() {
        return que.empty();
    }
};

Leetcode: Min Stack

 Min Stack

My Submissions

Total Accepted: 57566 Total Submissions: 271279 Difficulty: Easy

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.

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 Stack Design

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--------------------------------------------------------------------------
Keep two stacks inside: one store all and the other store the minimal.
Note: only the current x is smaller or equal to the mimStack.top() is saved.



///////////////////////////////////////////////////////////
class MinStack {

public:
    void push(int x) {
        curSta.push(x);
        if(minSta.empty() || x<=minSta.top() )
           minSta.push(x);
    }

    void pop() {
       
        if(minSta.top() == curSta.top() )
            minSta.pop();
        curSta.pop(); //Note: this line should be put in last, otherwise, the above codes can not be compared!!!!       
    }

    int top() {
        return curSta.top();
    }

    int getMin() {
        return minSta.top();
    }
   
private:
    stack<int> minSta;
    stack<int> curSta;  
};