20. 有效的括号

栈

import java.util.Scanner;
import java.util.Deque;

public class Algorithm {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.println(new Algorithm().isValid(scanner.nextLine()));
    }

    public boolean isValid(String s) {

        /**
     	 * Java推荐使用Deque类来实现栈（虽然也会破坏封装）
    	 */
        Deque<Character> stack = new ArrayDeque<>();

        for (int i = 0; i < s.length(); i++) {

            char c = s.charAt(i);

            /**
     		 * 只将左括号入栈，右括号用来和栈顶元素进行比较
    		 */
            if (c == '(' || c == '[' || c == '{') {
                stack.push(c);
            }

            else {

                if (stack.isEmpty()) {
                    return false;
                }

                char top = stack.pop();

                if (c == ')' && top != '(') {
                    return false;
                }

                if (c == ']' && top != '[') {
                    return false;
                }

                if (c == '}' && top != '{') {
                    return false;
                }
            }
        }
        
        /**
         * 如果括号全部匹配，栈应该为空
    	 */
        return stack.isEmpty();
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

https://leetcode-cn.com/problems/valid-parentheses/