[LeetCode] 232. Implement Queue using Stacks Java

標籤：logs   操作   iat   stand   note   bsp   and   sem   ble   

題目：

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

 

題意及分析：使用stack實現queue的功能。這裡對於queue的push和enpty操作來說，和stack一樣就行。對於pop和peek，由於stack是pop和peek都是從隊尾來講，然而queue是要的這兩個操作時對於隊首的元素來講。所以這裡需要使用另一個stack來講當前stack倒序添加進新stack,對於最後一個元素 取值或者去掉，然後在將新stack倒序回去即可。具體看代碼：

代碼：

public class MyQueue {    Stack<Integer> stack;    /** Initialize your data structure here. */    public MyQueue() {        stack = new Stack<>();    }        /** Push element x to the back of queue. */    public void push(int x) {    stack.push(x);        }        /** Removes the element from in front of queue and returns that element. */    public int pop() {    Stack<Integer> tempStack=new Stack<>();    int lenght=stack.size();    for(int i=0;i<lenght-1;i++){    tempStack.push(stack.peek());    stack.pop();    }    int res=stack.peek();    stack.pop();    int tempSize=tempStack.size();    for(int i=0;i<tempSize;i++){    stack.push(tempStack.peek());    tempStack.pop();    }    return res;    }        /** Get the front element. */    public int peek() {    Stack<Integer> tempStack=new Stack<>();    int lenght=stack.size();    for(int i=0;i<lenght-1;i++){    tempStack.push(stack.peek());    stack.pop();    }    int res=stack.peek();    stack.pop();    tempStack.push(res);    int tempSize=tempStack.size();    for(int i=0;i<tempSize;i++){    stack.push(tempStack.peek());    tempStack.pop();    }    return res;    }        /** Returns whether the queue is empty. */    public boolean empty() {        return stack.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] 232. Implement Queue using Stacks Java