#A：门牌制作 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);
}#寻找 2020 16520
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner 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： 蛇形填数 761
解题思路：先手写几行就可以发现，第一行第一列的值为1，第二行第二列的值为5.第三行第三列的值为13，第四行第四列的值为25，仔细观察就可以发现第i行第i列的值为 a = a + (i*4)。
public static void main(String[] args) {
// TODO Auto-generated method stub
int sum = 1;
for (int i = 0; i < 20; i++) {
sum = sum + (i * 4);
}System.out.println(sum);
}#D：七段码（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() {
//存储七位数的关系
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：成绩分析（模拟、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：单词分析（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： 数字三角形（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) = a1
int[][] 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（规律题吧）
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: 装饰珠
