LeetCode225——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.

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.

实现：

class Stack {

public:

// Push element x onto stack.

void push(int x) {

dq.push_back(x);

last = x;

}

// Removes the element on top of the stack.

void pop() {

if (empty()) return;

vector<int> vec;

while (!empty()) {

vec.push_back(dq.front());

dq.pop_front();

}

for (int i = 0; i < vec.size()-1; i++) {

push(vec[i]);

}

}

// Get the top element.

int top() {

return last;

}

// Return whether the stack is empty.

bool empty() {

return dq.size() == 0;

}

private:

deque<int> dq;

int last;

};

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