【代码】Java实现归并排序

前言

Java实现归并排序

正序

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void mergeSort(int[] arr, int left, int right) {
if (left < right) {
int mid = left + (right - left) / 2;
mergeSort(arr, left, mid);
mergeSort(arr, mid + 1, right);
merge(arr, left, mid, right);
}
}

void merge(int[] arr, int left, int mid, int right) {
int n1 = mid + 1 - left;
int n2 = right - mid;
int[] leftArr = new int[n1];
int[] rightArr = new int[n2];
for (int i = 0; i < n1; i++) {
leftArr[i] = arr[left + i];
}
for (int j = 0; j < n2; j++) {
rightArr[j] = arr[mid + 1 + j];
}
int i = 0, j = 0;
int k = left;
while (i < n1 && j < n2) {
if (leftArr[i] <= rightArr[j]) {
arr[k] = leftArr[i];
i++;
} else {
arr[k] = rightArr[j];
j++;
}
k++;
}
while (i < n1) {
arr[k] = leftArr[i];
i++;
k++;
}
while (j < n2) {
arr[k] = rightArr[j];
j++;
k++;
}
}

void sortASC(int[] nums) {
mergeSort(nums, 0, nums.length - 1);
}

倒序

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void mergeSort(int[] arr, int left, int right) {
if (left < right) {
int mid = left + (right - left) / 2;
mergeSort(arr, left, mid);
mergeSort(arr, mid + 1, right);
merge(arr, left, mid, right);
}
}

void merge(int[] arr, int left, int mid, int right) {
int n1 = mid + 1 - left;
int n2 = right - mid;
int[] leftArr = new int[n1];
int[] rightArr = new int[n2];
for (int i = 0; i < n1; i++) {
leftArr[i] = arr[left + i];
}
for (int j = 0; j < n2; j++) {
rightArr[j] = arr[mid + 1 + j];
}
int i = 0, j = 0;
int k = left;
while (i < n1 && j < n2) {
if (leftArr[i] >= rightArr[j]) {
arr[k] = leftArr[i];
i++;
} else {
arr[k] = rightArr[j];
j++;
}
k++;
}
while (i < n1) {
arr[k] = leftArr[i];
i++;
k++;
}
while (j < n2) {
arr[k] = rightArr[j];
j++;
k++;
}
}

void sortDESC(int[] nums) {
mergeSort(nums, 0, nums.length - 1);
}

完成