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递归:
void __merge(int arr[], int l, int mid, int r) {
int a = r - l + 1;
int *aux = new int[a];
for (int i = l; i <= r; i++)
aux[i - l] = arr[i];
int i = l, j = mid + 1;
for (int k = l; k <= r; k++) {
if (i > mid){
arr[k] = aux[j - l];
j++;
}
else if (j > r) {
arr[k] = aux[i - l];
i++; }
else if (aux[i - l] < aux[j - l]) {
arr[k] = aux[i - l];
i++; }
else {
arr[k] = aux[j - l];
j++; }
}
delete aux;
}
void __mergeSort(int arr[], int l, int r) { // 递归调用
if (l >= r)
return;
int mid = (l + r) / 2;
__mergeSort(arr, l, mid);
__mergeSort(arr, mid+1, r);
if( arr[mid] > arr[mid+1]) // 减少不必要的合并
__merge(arr, l, mid, r);
}
void mergeSort(int arr[], int n) {
__mergeSort(arr, 0, n - 1);
}
自底向上:
void mergeSortBU(int arr[], int n) {
for (int sz = 1; sz <= n; sz += sz)
for (int i = 0; i + sz < n; i += sz + sz)
__merge(arr, i, i + sz - 1, min(i + sz + sz - 1, n - 1));
}