| 32 | } |
| 33 | |
| 34 | public static void main(String[] args) { |
| 35 | // 随机生成长度为n |
| 36 | // 值在0~v-1之间 |
| 37 | // 且任意相邻两数不相等的数组 |
| 38 | int n = 10; |
| 39 | int v = 4; |
| 40 | int[] arr1 = new int[n]; |
| 41 | arr1[0] = (int) (Math.random() * v); |
| 42 | for (int i = 1; i < n; i++) { |
| 43 | do { |
| 44 | arr1[i] = (int) (Math.random() * v); |
| 45 | } while (arr1[i] == arr1[i - 1]); |
| 46 | } |
| 47 | for (int num : arr1) { |
| 48 | System.out.print(num + " "); |
| 49 | } |
| 50 | System.out.println(); |
| 51 | System.out.println("========="); |
| 52 | |
| 53 | // java中的动态数组是ArrayList |
| 54 | // 各个语言中的动态数组的初始大小和实际扩容因子可能会变化，但是均摊都是O(1) |
| 55 | // 课上用2作为扩容因子只是举例而已 |
| 56 | ArrayList<Integer> arr2 = new ArrayList<>(); |
| 57 | arr2.add(5); // 0 |
| 58 | arr2.add(4); // 1 |
| 59 | arr2.add(9); // 2 |
| 60 | arr2.set(1, 6); // arr[1]由4改成了6 |
| 61 | System.out.println(arr2.get(1)); |
| 62 | System.out.println("========="); |
| 63 | |
| 64 | int[] arr = { 64, 31, 78, 0, 5, 7, 103 }; |
| 65 | bubbleSort(arr); |
| 66 | for (int num : arr) { |
| 67 | System.out.print(num + " "); |
| 68 | } |
| 69 | System.out.println(); |
| 70 | System.out.println("========="); |
| 71 | |
| 72 | int N = 200000; |
| 73 | long start; |
| 74 | long end; |
| 75 | System.out.println("测试开始"); |
| 76 | start = System.currentTimeMillis(); |
| 77 | for (int i = 1; i <= N; i++) { |
| 78 | for (int j = i; j <= N; j += i) { |
| 79 | // 这两个嵌套for循环的流程，时间复杂度为O(N * logN) |
| 80 | // 1/1 + 1/2 + 1/3 + 1/4 + 1/5 + ... + 1/n，也叫"调和级数"，收敛于O(logN) |
| 81 | // 所以如果一个流程的表达式 : n/1 + n/2 + n/3 + ... + n/n |
| 82 | // 那么这个流程时间复杂度O(N * logN) |
| 83 | } |
| 84 | } |
| 85 | end = System.currentTimeMillis(); |
| 86 | System.out.println("测试结束，运行时间 : " + (end - start) + " 毫秒"); |
| 87 | |
| 88 | System.out.println("测试开始"); |
| 89 | start = System.currentTimeMillis(); |
| 90 | for (int i = 1; i <= N; i++) { |
| 91 | for (int j = i; j <= N; j++) { |