update

src/栈相关/q232_用栈实现队列/f1/MyQueue.java

@@ -0,0 +1,51 @@
+package 栈相关.q232_用栈实现队列.f1;
+
+import java.util.Stack;
+
+/**
+ * 双栈 入队o(n) 出队o(1)
+ */
+class MyQueue {
+
+    private Stack<Integer> s1 = new Stack<>();
+    private Stack<Integer> s2 = new Stack<>();
+    private Integer front;
+
+    /** Initialize your data structure here. */
+    public MyQueue() {
+
+    }
+
+    /** Push element x to the back of queue. */
+    public void push(int x) {
+        if (s1.empty()){
+            front = x;
+        }
+        while (!s1.isEmpty()){
+            s2.push(s1.pop());
+        }
+        s2.push(x);
+        while (!s2.isEmpty()){
+            s1.push(s2.pop());
+        }
+    }
+
+    /** Removes the element from in front of queue and returns that element. */
+    public int pop() {
+        int value = s1.pop();
+        if (!s1.empty()){
+            front = s1.peek();
+        }
+        return value;
+    }
+
+    /** Get the front element. */
+    public int peek() {
+        return front;
+    }
+
+    /** Returns whether the queue is empty. */
+    public boolean empty() {
+        return s1.isEmpty();
+    }
+}

src/栈相关/q232_用栈实现队列/f2/MyQueue.java

@@ -0,0 +1,49 @@
+package 栈相关.q232_用栈实现队列.f2;
+
+import java.util.Stack;
+
+/**
+ * 双栈 入队o(1) 出队平均o(1)，最坏o(n)
+ */
+class MyQueue {
+
+    private Stack<Integer> s1 = new Stack<>();
+    private Stack<Integer> s2 = new Stack<>();
+    private Integer front;
+
+    /** Initialize your data structure here. */
+    public MyQueue() {
+
+    }
+
+    /** Push element x to the back of queue. */
+    public void push(int x) {
+        if (s1.empty()){
+            front = x;
+        }
+        s1.push(x);
+    }
+
+    /** Removes the element from in front of queue and returns that element. */
+    public int pop() {
+        if (s2.isEmpty()) {
+            while (!s1.isEmpty()){
+                s2.push(s1.pop());
+            }
+        }
+        return s2.pop();
+    }
+
+    /** Get the front element. */
+    public int peek() {
+        if (!s2.isEmpty()) {
+            return s2.peek();
+        }
+        return front;
+    }
+
+    /** Returns whether the queue is empty. */
+    public boolean empty() {
+        return s1.isEmpty() && s2.isEmpty();
+    }
+}