As the title described, you should only use two stacks to implement a queue's actions.
The queue should support push(element)
, pop()
and top()
where pop is pop the first(a.k.a front) element in the queue.
Both pop and top methods should return the value of first element.
分析
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public class Queue {
private Stack<Integer> stack1 = new Stack<Integer>();
private Stack<Integer> stack2 = new Stack<Integer>();
public Queue() {
// do initialization if necessary
}
public void push( int element) {
// write your code here
while (!stack1.empty()){
stack2.push(stack1.pop());
}
stack1.push(element);
while (!stack2.empty()){
stack1.push(stack2.pop());
}
}
public int pop() {
// write your code here
if (!stack1.empty())
return stack1.pop();
else
return - 1 ;
}
public int top() {
// write your code here
if (!stack1.empty())
return stack1.peek();
else
return - 1 ;
}
} |