day10:第五章 栈与队列part01|232.用栈实现队列|225. 用队列实现栈|20. 有效的括号|1047. 删除字符串中的所有相邻重复项

栈：用stack

• 进栈(push), 出栈(pop)

queue：用linkedList: offer(). poll()

 

232.用栈实现队列

class MyQueue {
    //用2个stack to implement the queue.
    //就是用第二个栈存数据。
    //为什么不用把out的数据再放回去？因为check了out不为空，就可以在out那里继续拿，in的可以一直存着，直到out为空，再把in的都转去out
    Stack<Integer> stackIn;
    Stack<Integer> stackOut;

    public MyQueue() {
        stackIn = new Stack<>();
        stackOut = new Stack<>();
    }
    
    public void push(int x) {
        stackIn.push(x);
    }
    
    public int pop() {
        reverseToOut();
        return stackOut.pop();
    }
    
    public int peek() {
        reverseToOut();
        return stackOut.peek();
    }
    
    public boolean empty() {
        return stackIn.isEmpty() && stackOut.isEmpty();
    }

    private void reverseToOut(){
        if(!stackOut.isEmpty()) return;
        while(!stackIn.isEmpty()){
            stackOut.push(stackIn.pop());
        }
    }
}

/**
 * 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();
 */

225. 用队列实现栈

Two queue

class MyStack {
    // two queue. b is temporary evert time x in, move all the element to b, add x to a, 
    // and move all the elements from b to a, so the first element in the a is the top in the stack
    Queue<Integer> a;
    Queue<Integer> b;
    public MyStack() {
        a = new LinkedList<>();
        b = new LinkedList<>();
    }
    
    public void push(int x) {
        while(!a.isEmpty()){
            b.offer(a.poll());
        }
        a.offer(x);
        while(!b.isEmpty()){
            a.offer(b.poll());
        }
    }
    
    public int pop() {
        return a.poll();
    }
    
    public int top() {
        return a.peek();
    }
    
    public boolean empty() {
        return a.isEmpty();
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */

one queue

class MyStack {
    // one queue: everty time when add the new element just add new and add the front to back.
    Queue<Integer> a;
    int size;
    public MyStack() {
        a = new LinkedList<>();
        size = 0;
    }
    
    public void push(int x) {
        a.offer(x);
        size++;
        int reduce = size;
        while (reduce-- > 1){
            a.offer(a.poll());
        }  
    }
    
    public int pop() {
        return a.poll();
    }
    
    public int top() {
        return a.peek();
    }
    
    public boolean empty() {
        return a.isEmpty();
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */

 

20. 有效的括号

class Solution {
    public boolean isValid(String s) {
        //如果是左边的，就把右边的放入queue,如果不是左边的就对比，对比不对或者为空，return false
        //其他的都是真，最后检查这个queue是不是空。
        //第一想法就是用map跟queue,但是这种做法更好。
        if(s.length() % 2 != 0) return false;
        Deque<Character> deque = new LinkedList<>();
        char ch;
        for(int i = 0; i < s.length(); i++){
            ch = s.charAt(i);
            if(ch == '('){
                deque.push(')');
            } else if(ch == '['){
                deque.push(']');
            } else if(ch == '{') {
                deque.push('}');
            } else if(deque.isEmpty() || deque.peek()!= ch ){
                return false;
            }else{
                deque.pop();
            }
        }
        return deque.isEmpty();
    }
}

 

1047. 删除字符串中的所有相邻重复项

class Solution {
    public String removeDuplicates(String s) {
        if (s == null) return null;
//直接存入stack然后对比。然后翻转string
        Stack<Character> stack = new Stack<>();
        for(int i = 0; i < s.length(); i++){
            char ch = s.charAt(i);
            if(!stack.isEmpty()){
                char tem = stack.peek();
                if(tem == ch){
                    stack.pop();
                }else{
                    stack.push(ch);
                }
            }else {
                stack.push(ch);
            }
        }
        String str = "";
        while(!stack.isEmpty()){
            str = stack.pop() + str;
        }
        return str;
    }
}