- 统计 整数型 `数组中小于 M 的数出现的个数;
//java
public class Histogram {
public static int[] histogram(int[] a, int M) {
int[] result = new int[M];
for (int i = 0; i < a.length; i++) {
if (a[i] >= 0 && a[i] < M) {
result[a[i]]++;
}
}
return result;
}
public static void main(String[] args) {
int[] a = { 1, 1, 2, 3, 1, 7, 5, 3, 2, 2, 2 };
int[] result = histogram(a, 11);
for (int i = 0; i < result.length; i++) {
System.out.printf("%3d", result[i]);
}
System.out.println();
}
}
输出:
0 3 4 2 0 1 0 1 0 0 0
0 出现的次数为 0 次
1 出现的次数为 3 次
2 出现的次数为 2 次
...
- a[i] = a[a[i]] 需要注意
int a[] = new int [10];
for(int i=0;i<10;i++){
a[i] = 9-i;
}
for(int i=0;i<10;i++){
a[i] = a[a[i]];
}
- 矩阵 matrix.length = 行 ; matrix[0].length = 列
//java
public class Ex13 {
public static void printTransposedMatrix(int[][] matrix) {
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[0].length; j++) {
System.out.printf("%6d", matrix[i][j]);
}
System.out.println();
}
System.out.println(matrix.length);
System.out.println(matrix[0].length);
for(int j =0;j<matrix[0].length;j++){
for(int i=0;i<matrix.length;i++){
System.out.printf("%6d",matrix[i][j]);
}
System.out.println();
}
}
public static void main(String[] args) {
int[][] a = { { 100, 200, 300 }, { 400, 500, 600 } };
printTransposedMatrix(a);
}
}
输出:
100 200 300
400 500 600
2
3
100 400
200 500
300 600
- 初认识递归
public static String exR1(int n) {
if (n <= 0) {
return "";
}
return exR1(n - 3) + n + exR1(n - 2) + n;
}
public static void main(String[] args) {
System.out.println(exR1(6));
}
输出为:
311361142246
5.乘法和乘方的递归实现
//java
public static int mystery(int a, int b) {
if (b == 0) {
return 0;
}
if (b % 2 == 0) {
return mystery(a + a, b / 2);
}
return mystery(a + a, b / 2) + a;
}
public static int mystery2(int a, int b) {
if (b == 0) {
return 1;
}
if (b % 2 == 0) {
return mystery2(a * a, b / 2);
}
return mystery2(a * a, b / 2) * a;
}
System.out.println(mystery(2, 25));
System.out.println(mystery(3, 11));
System.out.println(mystery2(2, 10));
System.out.println(mystery2(3, 4));
输出:
50
33
1024
81
- 斐波那契数
//java
public class Ex19 {
public static long[] F(int N) {
long[] fibonacci = new long[N + 1];
if (N == 0) {
return fibonacci;
}
fibonacci[1] = 1;
if (N == 1) {
return fibonacci;
}
for (int i = 2; i <= N; i++) {
fibonacci[i] = fibonacci[i - 1] + fibonacci[i - 2];
}
return fibonacci;
}
public static void main(String[] args) {
long[] fibonacci = F(100);
for (int N = 0; N < fibonacci.length; N++) {
System.out.println(N + " " + fibonacci[N]);
}
}
}
评价: 速度很快
//java
public class Fibonacci {
public static long F(int N) {
if (N == 0) {
return 0;
}
if (N == 1) {
return 1;
}
return F(N - 1) + F(N - 2);
}
public static void main(String[] args) {
for (int N = 0; N < 100; N++) {
System.out.println(N + " " + F(N));
}
// After 1 hour, the N is about 55-56
}
}
速度很慢
交换
void swap(int *a, int *b){
int tmp ;
tmp = *a;
*a = *b;
*b = tmp;
}
void swap2(int &a, int &b){
int tmp = a;
a = b;
b = tmp;
}
使用时
int x= 3;
int y = 5;
swap(&x,&y)
swap2(x,y)
