LeetCode-Implement Queue using Stacks

这种都是用两个stack倒来倒去 注意stack peek和pop的不同 注意返回值

class MyQueue {

private Stack<Integer> s1 = new Stack<Integer>();
private Stack<Integer> s2 = new Stack<Integer>();
// Push element x to the back of queue.
public void push(int x) {
s1.push(x);
}

// Removes the element from in front of queue.
public void pop() {
if ( s2.isEmpty()){
while ( !s1.isEmpty() ){
s2.push( s1.pop() );
}
}
s2.pop();
}

// Get the front element.
public int peek() {
if ( s2.isEmpty()){
while ( !s1.isEmpty() ){
s2.push( s1.pop() );
}
}
return s2.peek();
}

// Return whether the queue is empty.
public boolean empty() {
if ( s1.empty() && s2.empty() )
return true;
else
return false;
}
}