归并排序算法采用分治方法,把两个有序表合并成一个新的有序表。归并排序是一种稳定的排序算法,即相等元素的顺序不会改变。
public class MergeSort { private void merge(int[] a , int begin, int mid, int end, int[] temp) { int i = begin, j = mid + 1; int m = mid, n = end; int k = 0; while(i <= m && j <= n) { if(a[i] < a[j]) { temp[k++] = a[i++]; } else { temp[k++] = a[j++]; } } while(i <= m) { temp[k++] = a[i++]; } while(j <= n) { temp[k++] = a[j++]; } for(int h = 0; h < k; h++) { a[begin + h] = temp[h]; } } public void mergeSort(int[] a, int begin, int end, int[] temp) { if(begin < end) { int mid = (begin + end) / 2; mergeSort(a, begin, mid, temp); mergeSort(a, mid + 1, end, temp); merge(a, begin, mid, end, temp); } } public static void main(String[] args) { int[] arrays = new int[]{7, 20, 23, 0, 4, 8, 12, 3, 18, 8}; int[] temps = new int[arrays.length]; MergeSort mergeSort = new MergeSort(); mergeSort.mergeSort(arrays, 0, arrays.length - 1, temps); for(int temp : arrays) { System.out.print(temp + ","); } } }
归并排序的最好、最坏和平均时间复杂试都是O(nlogn),空间复杂度是O(n)。