LeetCode232——Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push
to top, peek/pop from top, size,
  and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.

• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

实现：

class Queue {

public:

// Push element x to the back of queue.

void push(int x) {

deq.push_back(x);

if (deq.size() == 1) {

front = x;

}

}

// Removes the element from in front of queue.

void pop(void) {

vector<int> vec;

while (deq.size() > 0) {

vec.push_back(deq.back());

deq.pop_back();

}

if (vec.size() > 0) {

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

push(vec[i]);

}

}

}

// Get the front element.

int peek(void) {

return front;

}

// Return whether the queue is empty.

bool empty(void) {

return deq.size() == 0;

}

private:

deque<int> deq;

int front;

};

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