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package class08;
public class Code02_MergeSort {
// 递归方法实现
public static void mergeSort1(int[] arr) {
if (arr == null || arr.length < 2) {
return;
}
process(arr, 0, arr.length - 1);
}
// arr[L...R]范围上，请让这个范围上的数，有序！
public static void process(int[] arr, int L, int R) {
if (L == R) {
return;
}
// int mid = (L + R) / 2
int mid = L + ((R - L) >> 1);
process(arr, L, mid);
process(arr, mid + 1, R);
merge(arr, L, mid, R);
}
public static void merge(int[] arr, int L, int M, int R) {
int[] help = new int[R - L + 1];
int i = 0;
int p1 = L;
int p2 = M + 1;
while (p1 <= M && p2 <= R) {
help[i++] = arr[p1] <= arr[p2] ? arr[p1++] : arr[p2++];
}
// 要么p1越界，要么p2越界
// 不可能出现：共同越界
while (p1 <= M) {
help[i++] = arr[p1++];
}
while (p2 <= R) {
help[i++] = arr[p2++];
}
for (i = 0; i < help.length; i++) {
arr[L + i] = help[i];
}
}
public static void mergeSort2(int[] arr) {
if (arr == null || arr.length < 2) {
return;
}
int step = 1;
int N = arr.length;
while (step < N) {
int L = 0;
while (L < N) {
int M = 0;
if (N - L >= step) {
M = L + step - 1;
} else {
M = N - 1;
}
if (M == N - 1) {
break;
}
int R = 0;
if (N - 1 - M >= step) {
R = M + step;
} else {
R = N - 1;
}
merge(arr, L, M, R);
if (R == N - 1) {
break;
} else {
L = R + 1;
}
}
if (step > N / 2) {
break;
}
step *= 2;
}
}
// 非递归方法实现
// public static void mergeSort2(int[] arr) {
// if (arr == null || arr.length < 2) {
// return;
// }
// int N = arr.length;
// int mergeSize = 1;
// while (mergeSize < N) {
// int L = 0;
// while (L < N) {
// if (mergeSize >= N - L) {
// break;
// }
// int M = L + mergeSize - 1;
// int R = M + Math.min(mergeSize, N - M - 1);
// merge(arr, L, M, R);
// L = R + 1;
// }
// if (mergeSize > N / 2) {
// break;
// }
// mergeSize <<= 1;
// }
// }
// for test
public static int[] generateRandomArray(int maxSize, int maxValue) {
int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
for (int i = 0; i < arr.length; i++) {
arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
}
return arr;
}
// for test
public static int[] copyArray(int[] arr) {
if (arr == null) {
return null;
}
int[] res = new int[arr.length];
for (int i = 0; i < arr.length; i++) {
res[i] = arr[i];
}
return res;
}
// for test
public static boolean isEqual(int[] arr1, int[] arr2) {
if ((arr1 == null && arr2 != null) || (arr1 != null && arr2 == null)) {
return false;
}
if (arr1 == null && arr2 == null) {
return true;
}
if (arr1.length != arr2.length) {
return false;
}
for (int i = 0; i < arr1.length; i++) {
if (arr1[i] != arr2[i]) {
return false;
}
}
return true;
}
// for test
public static void printArray(int[] arr) {
if (arr == null) {
return;
}
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " ");
}
System.out.println();
}
// for test
public static void main(String[] args) {
int testTime = 500000;
int maxSize = 100;
int maxValue = 100;
System.out.println("测试开始");
for (int i = 0; i < testTime; i++) {
int[] arr1 = generateRandomArray(maxSize, maxValue);
int[] arr2 = copyArray(arr1);
mergeSort1(arr1);
mergeSort2(arr2);
if (!isEqual(arr1, arr2)) {
System.out.println("出错了！");
printArray(arr1);
printArray(arr2);
break;
}
}
System.out.println("测试结束");
}
}