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题目链接: https://leetcode.com/problems/implement-stack-using-queues/
GitHub答案源代码请点击这里
解法1: 优化add()
class MyStack {
Queue<Integer> q1, q2;
/** Initialize your data structure here. */
public MyStack() {
q1 = new LinkedList<>();
q2 = new LinkedList<>();
}
/** Push element x onto stack. */
public void push(int x) {
if (!q2.isEmpty()) {
q1.add(q2.poll());
}
q1.add(x);
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
if (!q2.isEmpty()) {
return q2.poll();
}
top();
return q2.poll();
}
/** Get the top element. */
public int top() {
if (!q2.isEmpty()) {
return q2.peek();
}
while (q1.size() > 1) {
q2.add(q1.poll());
}
Queue<Integer> tmp = q1;
q1 = q2;
q2 = tmp;
return q2.peek();
}
/** Returns whether the stack is empty. */
public boolean empty() {
return q1.isEmpty() && q2.isEmpty();
}
}
解法2: 优化pop()
class MyStack {
Queue<Integer> q1, 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.add(x);
while (!q2.isEmpty()) {
q1.add(q2.poll());
}
Queue<Integer> tmp = q1;
q1 = q2;
q2 = tmp;
}
/** 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 q1.isEmpty() && q2.isEmpty();
}
}