#A：門牌制作?624
解一：
public?static?void?main(String[] args) {int?count=0;for(int?i=1;i<=2020;i++) {int?n=i;while(n>0) {if(n%10==2) {count++;}n/=10;}}System.out.println(count);}解二：?
public?static?void?main(String[] args) {int?cnt=0;for?(int?i?= 1; i?<= 2020; i++) {String s=i+"";for?(int?j?= 0; j?< s.length(); j++) {if(s.charAt(j)=='2') cnt++;}}System.out.println(cnt);}#尋找 2020??16520
public static void main(String[] args) {// TODO Auto-generated method stubScanner scanner = new Scanner(System.in);char[][] ch = new char[300][300];long sum = 0 ;for (int i = 0; i < ch.length; i++) {ch[i] = scanner.nextLine().toCharArray();}for (int i = 0; i < ch.length; i++) {for (int j = 0; j < ch[0].length; j++) {if (j+3<ch[0].length) {if (ch[i][j]=='2' && ch[i][j+1]=='0' &&ch[i][j+2]=='2' &&ch[i][j+3]=='0' ) {sum++;}}if (i+3<ch.length) {if (ch[i][j]=='2' && ch[i+1][j]=='0' &&ch[i+2][j]=='2' &&ch[i+3][j]=='0') {sum++;}}if (i+3<ch.length && j+3<ch[0].length) {if (ch[i][j]=='2' && ch[i+1][j+1]=='0' &&ch[i+2][j+2]=='2' &&ch[i+3][j+3]=='0') {sum++;}}}}System.out.println(sum);//16520}#C： 蛇形填數?761
解題思路：先手寫幾行就可以發現，第一行第一列的值為1，第二行第二列的值為5.第三行第三列的值為13，第四行第四列的值為25，仔細觀察就可以發現第i行第i列的值為? a = a + (i*4)。
public?static?void?main(String[] args) {// TODO?Auto-generated method stubint?sum?= 1;for?(int?i?= 0; i?< 20; i++) {sum?= sum?+ (i?* 4);}System.out.println(sum);}#D：七段碼（dfs) ?80
? 解一：
static?int[][] list?= new?int[][]{{0,1,0,0,0,1,0},{1,0,1,0,0,0,1},{0,1,0,1,0,0,1},{0,0,1,0,1,0,0},{0,0,0,1,0,1,1},{1,0,0,0,1,0,1},{0,1,1,0,1,1,0},};static?Set<Set<Integer>> set?= new?HashSet<>();static?boolean[] booleans?= new?boolean[7];public?static?void?main(String[] args) {Scanner sc?= new?Scanner(System.in);ArrayList<Integer> arr?= new?ArrayList<>();for?(int?i?= 0 ; i?< 7 ; i++){booleans[i] = true;arr.add(i);set.add(new?HashSet<>(arr));dfs(arr);booleans[i] = false;arr.remove(arr.size()-1);}System.out.println(set.size());}private?static?void?dfs(ArrayList<Integer> arr) {set.add(new?HashSet<>(arr));int?last?= arr.get(arr.size()-1);for?(int?i?= 0 ; i?< list[last].length?; i++){if?(list[last][i] == 1 && !booleans[i]){booleans[i] = true;arr.add(i);dfs(arr);booleans[i] = false;arr.remove(arr.size()-1);}}}? 解二：
public static void main(String[] args) {creatGraph();dfs(1);System.out.println(count);}private static void creatGraph() {arr[1][2] = arr[1][6] = 1;arr[2][1] = arr[2][7] = arr[2][3] = 1;arr[3][2] = arr[3][4] = arr[3][7] = 1;arr[4][3] = arr[4][5] = 1;arr[5][4] = arr[5][6] = arr[5][7] = 1;arr[6][1] = arr[6][5] = arr[6][7] = 1;}private static void dfs(int k) {if (k > 7) {check();int q = 0;for (int i = 1; i <= 7; i++) {if (v[i] && res[i] == i) {q++;}}if (q == 1)count++;return;}v[k] = true;dfs(k + 1);v[k] = false;dfs(k + 1);}private static void check() {for (int i = 1; i <= 7; i++) {res[i] = i;}for (int i = 1; i <= 7; i++) {for (int j = 1; j <= 7; j++) {if (arr[i][j] != 0 && v[i] && v[j]) {int fx = find(i), fy = find(j);if (fx != fy) {res[fx] = fy;}}}}}private static int find(int p) {if (res[p] == p) {return p;} else {res[p] = find(res[p]);return res[p];}}解三：
static ArrayList<Integer>[] list;static HashSet<Integer> set = new HashSet<Integer>();public static void main(String[] args) {init();for(int i=0; i<7; ++i) {vis[0] = i;set.add(1<<i);dfs(1, 1<<i);}System.out.println(set.size());}static int[] vis = new int[7];public static void dfs(int n, int v) {for(int i=0; i<n; ++i) {for(int t : list[vis[i]]) {int p = v|(1<<t);if(!set.contains(p)) {set.add(p);vis[n] = t;dfs(n+1, p);}}}}public static void init() {//存儲七位數的關系 list = new ArrayList[7];for(int i=0; i<7; ++i) {list[i] = new ArrayList<Integer>();}list[0].add(1);list[0].add(5);list[1].add(0);list[1].add(6);list[1].add(2);list[2].add(1);list[2].add(3);list[2].add(6);list[3].add(2);list[3].add(4);list[4].add(3);list[4].add(5);list[4].add(6);list[5].add(0);list[5].add(4);list[5].add(6);list[6].add(1);list[6].add(2);list[6].add(4);list[6].add(5);}#E：排序??jonmlkihgfedcba
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#F：成績分析（模擬、暴力）
public static void main(String[] args) {Scanner sc=new Scanner(System.in);int n=sc.nextInt();int a,max=0,min=100;double sum=0;for(int i=0;i<n;i++) {a=sc.nextInt();if(max<a) {max=a;}if(min>a) {min=a;}sum+=a;}System.out.println(max);System.out.println(min);System.out.printf("%.2f",sum/n);}#G：單詞分析（字符串）
?
public?static?void?main(String[] args) {Scanner sc=new?Scanner(System.in);String str=sc.next();int[]arr?=new?int[26];int?a=0;;char?ch?= 'a';for(int?i=0;i<str.length();i++) {arr[str.charAt(i)-97]++;if(arr[str.charAt(i)-97]>a) {a=arr[str.charAt(i)-97];ch=str.charAt(i);}else?if(arr[str.charAt(i)-97]==a) {if(ch-str.charAt(i)>0) {a=arr[str.charAt(i)-97];ch=str.charAt(i);}}}System.out.println(ch); System.out.println(a);}#H： 數字三角形（DP）
???
?public?static?void?main(String[] args) {Scanner scanner?= new?Scanner(System.in);int?n=scanner.nextInt();int?[][]arr=new?int[n+1][n+1];for?(int?i?= 1; i?< arr.length; i++) {for?(int?j?= 1; j?<= i; j++) {int?s=scanner.nextInt();arr[i][j]=s+Math.max(arr[i-1][j-1],arr[i-1][j]);}}System.out.println(n%2==1?arr[n][n/2+1]:Math.max(arr[n][n/2],arr[n][n/2+1]));}}? 解二：
??public?static?void?main(String[] args) {Scanner scanner?= new?Scanner(System.in);int?n?= scanner.nextInt();//base case : f(1) = a1int[][] dp?= new?int[n][n+1];int?temp?= scanner.nextInt();dp[0][0] = temp;int?max?= Integer.MIN_VALUE;for?(int?i?= 2; i?<= n; i++) {for?(int?j?= 0; j?< i; j++) {temp?= scanner.nextInt();if?(j?!= 0 && j?!= (i-1)) {dp[i-1][j] = Integer.max(dp[i-2][j-1],dp[i-1][j])+temp;}else?if?(j?== 0) {dp[i-1][j] = dp[i-2][j]+temp;}else?{dp[i-1][j] = dp[i-2][j-1]+temp;}if(dp[i-1][j] > max)max?= dp[i-1][j];}dp[i-1][n] = max;max?= Integer.MIN_VALUE;}System.out.println(dp[n-1][n]);scanner.close();}#?I：子串分值和（規律題吧）
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解一：???
?public?static?void?main(String[] args) {Scanner sc?= new?Scanner(System.in);int[] arr?= new?int[26];long?res?= 0;for(int?i?= 0;i?< 26;i++){arr[i] = -1;}String s?= sc.nextLine();String[] str?= s.split("");int?willIndex?= str.length;for(int?i?= 0;i?< str.length;i++){int?lastIndex?= arr[str[i].charAt(0)-97];res?+= (long)(i?- lastIndex)*(willIndex?- i);arr[str[i].charAt(0)-97] = i;}System.out.println(res);}? 解二：
public?static?void?main(String[] args) {Scanner sc?= new?Scanner(System.in);String str?= sc.next();int[] nums?= new?int[26];long?res?= 0;long?n?= str.length();str?= "0"+str;for?(int?i?= 1; i?< str.length(); i++) {res?+= (i?- nums[str.charAt(i) - 'a']) * (n?- i+1);nums[str.charAt(i) - 'a'] = i;}System.out.println(res);}#J: 裝飾珠
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