2.栈

import java.util.Stack;

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        for(int i = 0;i < s.length();i ++){
            char c = s.charAt(i);
            if(c == ‘(‘ || c == ‘[‘ || c == ‘{‘)
                stack.push(c);
            else{
                if(stack.isEmpty())
                    return false;
                char topChar = stack.pop();
                if(c == ‘)‘ && topChar != ‘(‘)
                    return false;
                if(c == ‘]‘ && topChar != ‘[‘)
                    return false;
                if(c == ‘}‘ && topChar != ‘{‘)
                    return false;
            }
        }
        return stack.isEmpty();
    }
}

public class Main {
    public static void main(String[] args) {
        Solution so = new Solution();
        String s = "(){]{}";
        boolean res = so.isValid(s);
        System.out.println(res);    //false
    }
}

public interface Stack<E> {
    int getSize();
    boolean isEmpty();
    void push(E e);
    E pop();
    E peek();
}

public class ArrayStack<E>  implements Stack<E>{
    Array<E> array;

    public ArrayStack(int capacity){
       array = new Array<>(capacity);
   }

    public ArrayStack(){
        array = new Array<>();
    }

    @Override
    public int getSize() {
        return array.getSize();
    }

    @Override
    public boolean isEmpty(){
        return array.isEmpty();
    }

    public int getCapacity(){
        return array.getCapacity();
    }

    @Override
    public void push(E e) {
        array.addLast(e);
    }

    @Override
    public E pop() {
        return array.removeLast();
    }

    @Override
    public E peek(){
        return array.getLast();
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Stack:");
        res.append("[");
        for(int i = 0;i < array.getSize();i ++){
            res.append(array.get(i));
            if(i != array.getSize() - 1)
                res.append(",");
        }
        res.append("] top");
        return res.toString();
    }
}

public class Main {
    public static void main(String[] args) {
        ArrayStack<Integer> stack = new ArrayStack<>();
        for(int i = 0;i < 5;i ++){
            stack.push(i);
            System.out.println(stack);
        }
        stack.pop();
        System.out.println(stack);
//        Stack:[0] top
//        Stack:[0,1] top
//        Stack:[0,1,2] top
//        Stack:[0,1,2,3] top
//        Stack:[0,1,2,3,4] top
//        Stack:[0,1,2,3] top
    }
}