#### 1.萬年歷
```
// 萬年歷,1900-1-1是週一。
public static void calendar() {
Scanner sc = new Scanner(System.in);
System.out.println("請輸入年份");
int year = sc.nextInt();
System.out.println("請輸入月份");
int month = sc.nextInt();
// 計算year年(month-1)月到1900-1-1總共有幾天。
int total = 0;
for (int i = 1900; i < year; i++) {
if (i % 400 == 0 || (i % 100 != 0 && i % 4 == 0)) {
total += 366;
} else {
total += 365;
}
}
for (int i = 1; i < month; i++) {
int days = 31;
switch (i) {
case 4:
case 6:
case 9:
case 11:
days = 30;
break;
case 2:
if (year % 400 == 0 || (year % 100 != 0 && year % 4 == 0)) {
days = 29;
} else {
days = 28;
}
}
total += days;
}
// year年month月的第一天是星期幾。
int firstDay = total % 7 + 1;
firstDay = (firstDay % 7 == 0) ? 0 : firstDay;
// 獲得month月的天數。
int currentMonthDays = 31;
switch (month) {
case 4:
case 6:
case 9:
case 11:
currentMonthDays = 30;
break;
case 2:
if (year % 400 == 0 || (year % 100 != 0 && year % 4 == 0)) {
currentMonthDays = 29;
} else {
currentMonthDays = 28;
}
}
int count = firstDay + currentMonthDays;
System.out.println("日\t一\t二\t三\t四\t五\t六");
for (int i = 0; i < count; i++) {
int j = i - firstDay + 1;
if (j < 1) {
System.out.print("\t");
} else {
System.out.print(j + "\t");
}
if ((i + 1) % 7 == 0) {
System.out.println();
}
}
}
```
#### 2.最大公約數&最小公倍數
`兩個數的乘積等于這兩個數的最大公約數與最小公倍數的乘積`
```
public class demo {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("請輸入第一個整數：");
int num1 = sc.nextInt();
System.out.print("請輸入第二個整數：");
int num2 = sc.nextInt();
/*if(num1>num2){
System.out.println(test(num2,num1));
}else{
System.out.println(test(num1,num2));
}*/
System.out.println(test1(num1,num2));
//最小公倍數
System.out.println((num1*num2)/test1(num1,num2));
}
//輾轉相除
public static int test(int a,int b){
if(a%b==0){
return b;
}else{
return test(b,a%b);
}
}
//更相減損
public static int test1(int a,int b){
if(a==b){
return a;
}else if(a<b){
return test1(b-a,a);
}else{
return test1(a-b,b);
}
}
}
```
#### 3.質數
```
public static void prime(){
Scanner sc = new Scanner(System.in);
System.out.println("請輸入一個整數:");
int num = sc.nextInt();
if(num==1){
System.out.println("不是一個質數");
}else{
double k = Math.sqrt(num);
int i = 2;
for(;i<=k;i++){
if(num%i==0){
break;
}
}
if(i<=k){
System.out.println("不是一個質數");
}else{
System.out.println("是一個質數");
}
}
}
```
#### 4.楊輝三角
```
public static void test() {
int[][] arr = new int[10][10];
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < i + 1; j++) {
if (j == 0) {
arr[i][j] = 1;
} else {
arr[i][j] = arr[i - 1][j - 1] + arr[i - 1][j];
}
}
}
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j] != 0) {
System.out.print(arr[i][j] + "\t");
}
}
System.out.println();
}
}
```
#### 5."小技巧"
```
//從控制臺循環的輸入數個正整數。輸入-1表示結束。 循環結束后，所有數的最大的數和最小的數。
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int max = 0, min = 0;
for (int i = 1;; i++) {
System.out.println("請輸入第" + i + "個數:");
int n = sc.nextInt();
if (n == -1)
break;
if (i == 1) {
max = n;
min = n;
}
max = n > max ? n : max;
min = n < min ? n : min;
}
System.out.println("最大數:" + max);
System.out.println("最小數:" + min);
}
```
#### 6.排序
```
//冒泡排序
public static void sort() {
int[] arr = { 1, 2, 3, 4, 55, -7 };
for (int i = 0; i < arr.length - 1; i++) {
for (int j = 0; j < arr.length - 1 - i; j++) {
if (arr[j + 1] < arr[j]) {
int temp = arr[j + 1];
arr[j + 1] = arr[j];
arr[j] = temp;
}
}
}
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i] + "\t");
}
}
```
#### 7.把數組中大于arr[0]的元素移動到其前，小于的移動到其后。
```
public static void demo() {
int[] arr = { 3, 5, 2, 8, 6, 7, 1};
int flagIndex = 0;
for (int i = 1; i < arr.length; i++) {
if(arr[i]>arr[flagIndex]){
for (int j = i; j > flagIndex; j--) {
int temp = arr[j];
arr[j]= arr[j-1];
arr[j-1] = temp;
}
flagIndex++;
}
}
System.out.println(Arrays.toString(arr));
}
```
#### 8.在str1中刪除str2，統計刪除str2個數和刪除str2后的str1.
```
public class Test {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("請輸入字符串:");
String str1 = sc.next();
System.out.print("請輸入你要刪除的字符串:");
String str2 = sc.next();
del(str1, str2);
}
public static void del(String str1, String str2) {
StringBuilder sb1 = new StringBuilder(str1);
int count = 0;
int len = str2.length();
int index = sb1.indexOf(str2);
while (index != -1) {
count++;
sb1.delete(index, index + len);
index = sb1.indexOf(str2);
// 不能用index = sb1.indexOf(str2,index+len);因為已經把li刪除了
}
System.out.println("刪除后:" + sb1);
System.out.println("刪除個數：" + count);
}
}
```
