在第一章的第一部分中，有哪些关键内容或概念需要重点关注吗？

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javaimport java.util.Stack;

public class Main { public static void main(String[] args) { String str1=((()))(); print(isValid(str1), maxLength1(str1), maxLength2(str1));

String str2=((())()); print(isValid(str2), maxLength1(str2), maxLength2(str2)); }

private static void print(boolean isValid, int maxLength1, int maxLength2) { System.out.println(isValid + , + maxLength1 + , + maxLength2); }

private static boolean isValid(String s) { Stack stack=new Stack(); for (char c : s.toCharArray()) { if (c=='(') { stack.push(c); } else if (stack.isEmpty() || stack.pop() !='(') { return false; } } return stack.isEmpty(); }

private static int maxLength1(String s) { int max=0; for (int i=0; i

private static int maxLength2(String s) { int max=0, count=0; for (int i=0; i 0) count--; else { max=Math.max(max, count); count=0; } } max=Math.max(max, count); return max; }}

gistfile1.txt

import java.util.Stack; public class Main{ public static void main(String[] args){ String str1 = "((())())"; print(isValid(str1),maxLength1(str1),maxLength2(str1)); String str2 = "(())(()(())"; print(isValid(str2),maxLength1(str2),maxLength2(str2)); String str3 = "()(()()("; print(isValid(str3),maxLength1(str3),maxLength2(str3)); } public static void print(boolean b,int maxLength1,int maxLength2){ System.out.println(b+"\t"+maxLength1+"\t"+maxLength2); } public static boolean isValid(String str){ if(str==null||str.equals("")) return false; char[] chars=str.toCharArray(); int status=0; for(int i=0;i stack=new Stack (); int start=-1,maxLength=0; for(int i=0;i =0&&chars[pre]=='(') dp[i]=dp[i-1]+2+(pre>0 ? dp[pre-1]:0); } maxLength=Math.max(maxLength,dp[i]); } return maxLength; } }