1.描述
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).
2.分析
3.代碼
class MyStack {
public:
/** Initialize your data structure here. */
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
q1.push(x);
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
if (empty()) exit(-1);
int ans;
if (!q1.empty()) {
ans = q1.back();
while (q1.size() > 1) {
q2.push(q1.front());
q1.pop();
}
q1.pop();
} else {
ans = q2.back();
while (q2.size() > 1) {
q1.push(q2.front());
q2.pop();
}
q2.pop();
}
return ans;
}
/** Get the top element. */
int top() {
if (empty()) exit(-1);
if (q1.size() >= 1)
return q1.back();
else if (q2.size() >= 1)
return q2.back();
}
/** Returns whether the stack is empty. */
bool empty() {
return q1.empty() && q2.empty();
}
private:
queue<int> q1, q2;
};
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* bool param_4 = obj.empty();
*/
