##225. 用队列实现栈
如题
###题解
在push时候搞点事情：push时入队1，在把队2的元素一个个入队1，再交换队2和队1，保持队1除pushguocheng 始终为空。
###代码
class MyStack {
private Queue<Integer> q1;
private Queue<Integer> q2;
/** Initialize your data structure here. */
public MyStack() {
q1=new LinkedList<>();
q2=new LinkedList<>();
}
/** Push element x onto stack. */
public void push(int x) {
q1.offer(x);
while(!q2.isEmpty()){
q1.offer(q2.poll());
}
Queue temp=q1;
q1=q2;
q2=temp;
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
return q2.poll();
}
/** Get the top element. */
public int top() {
return q2.peek();
}
/** Returns whether the stack is empty. */
public boolean empty() {
return q2.isEmpty();
}
}
/**
* 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();
* boolean param_4 = obj.empty();
*/
剑指 Offer 09. 用两个栈实现队列
###题解
入队直接入栈1
出队：若栈2有元素直接出栈，否则若栈1为空则返回-1，若不为空则全部入栈2并弹出栈顶元素。
###代码
class CQueue {
LinkedList s1,s2;
public CQueue() {
s1 = new LinkedList<>();
s2 = new LinkedList<>();
}
public void appendTail(int value) {
s1.add(value);
}
public int deleteHead() {
if(!s2.isEmpty()){
return s2.removeLast();
}else if(s1.isEmpty()){
return -1;
}else{
while(!s1.isEmpty()){
int val=s1.removeLast();
s2.add(val);
}
return s2.removeLast();
}
}
}
/**
- Your CQueue object will be instantiated and called as such:
- CQueue obj = new CQueue();
- obj.appendTail(value);
- int param_2 = obj.deleteHead();
*/