棧[232] 用棧實現隊列

使用棧實現隊列的下列操作：

push(x) – 將一個元素放入隊列的尾部。
pop() – 從隊列首部移除元素。
peek() – 返回隊列首部的元素。
empty() – 返回隊列是否爲空。

方法1：輔助棧

思路：
用棧模擬隊列的關鍵是保證新元素位於棧底（即讓新元素排在隊尾）。
設有棧s1，s2，s1爲數據棧，s2爲輔助棧。新元素來得時候，將s1中的元素倒入s2，再將新元素入棧s1，最後再從s2中將元素放回s1，實現新元素排在隊尾的目的。

import java.util.Queue;
import java.util.Stack;

//leetcode submit region begin(Prohibit modification and deletion)
class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<>(); // 數據棧
        stack2 = new Stack<>(); // 輔助棧
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        while (!stack1.isEmpty()){ // s1中數據轉入s2
            int temp = stack1.pop();
            stack2.push(temp);
        }
        stack1.push(x); // s1中入棧新元素
        while (!stack2.isEmpty()){ // 將s2中的元素放回s1
            int temp = stack2.pop();
            stack1.push(temp);
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return stack1.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        return stack1.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */
//leetcode submit region end(Prohibit modification and deletion)

方法2：輔助棧

思路：
通過將s1中的數據倒入s2，實現s1中數據順序的逆轉，從而使得s1中的棧底元素成爲s2的棧頂元素。當s2非空時，出隊操作即爲s2的出棧操作；當s2爲空時，將s1的數據倒入s2，再對s2進行出棧操作。

import java.util.Queue;
import java.util.Stack;

//leetcode submit region begin(Prohibit modification and deletion)
class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;
    int front; // 保存s1的棧底元素
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<>(); // 數據棧
        stack2 = new Stack<>(); // 輔助棧
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        if (stack1.isEmpty()){
            front = x; // 防止s2爲空時，丟失隊首元素
        }
        stack1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (stack2.isEmpty()){
            while (!stack1.isEmpty()){
                stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        if (stack2.isEmpty()){
            return front;
        }
        return stack2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty()&& stack2.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */
//leetcode submit region end(Prohibit modification and deletion)