[数据结构与算法]LeetCode 数据结构入门 Day9 栈 / 队列 Java

20. 有效的括号

问题描述:

?代码:

class Solution {
    public boolean isValid(String s) {
       char []ss = new char[s.length()];
       ss=s.toCharArray();
       Map<Character,Character> hx = new HashMap<>();
       hx.put(')','(');
       hx.put(']','[');
       hx.put('}','{');

       Deque<Character> deque = new LinkedList<>();
       
       for(int i=0;i<ss.length;i++){
           if(hx.containsKey(ss[i])){
               if(deque.isEmpty()||hx.get(ss[i])!=deque.peek())
                    return false;
                deque.poll();
           }else{
                deque.push(ss[i]);
           }
       }
        return deque.isEmpty();
    }
}

思路:

?232. 用栈实现队列

问题描述:

?代码:

class MyQueue {
    Deque<Integer> s1;
    Deque<Integer> s2;
    public MyQueue() {
        s1= new LinkedList<Integer>();
        s2 = new LinkedList<Integer>();
    }
    
    public void push(int x) {  
            s1.push(x);
    }
    
    public int pop() {
            if(s2.isEmpty()){
                 while(!s1.isEmpty()){
                s2.push(s1.pop());
            }
            }
           
            int front=s2.pop();
         
            return front;
    }
    
    public int peek() {
        
        if(s2.isEmpty()){
            while(!s1.isEmpty()){
                s2.push(s1.pop());
            }
        }
            
            int front=s2.peek();
            return front;
    }
    
    public boolean empty() {
            if(s1.isEmpty()&&s2.isEmpty()){
                return true;
            }
            return false;
    }
}

/**
 * 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();
 */

思路:

利用堆栈和队列的性质