Merge Sort 的递归实现, 同时也是算法里面常见的分而治之方法。
先放上一段分而治之的算法定义, 一张图胜过千言万语。
- sort 整个数组 = sort 前半个数组, then sort后半个数组, then merge两个有序数组。
- merge整个有序数组是非递归程序,输入有array本身,left, middle 和 right
- 左半数组长度,有半数组长度
- 初始化两个新数组
- 两个新数组比较,比较结果更新到原来的数组里
- 比较完以后,有一个数组可能有碎片,将碎片加到原数组中
下面是实现代码:
public class MergeSortV1 { public void PrintArray(int[] array) { for (int i = 0; i < array.Length; i++) { Console.WriteLine(array[i]); } } public void Sort(int[] array, int left, int right) { if (left < right) { int middle = (right + left) / 2; Sort(array, left, middle); Sort(array, middle + 1, right); Merge(array, left, middle, right); } } public void Merge(int[] array, int left, int middle, int right) { int n1 = middle - left + 1; int n2 = right - middle; int[] L = new int[n1]; int[] R = new int[n2]; for(int i = 0; i < n1; i++) { L[i] = array[middle - left + i]; } for(int i = 0; i < n2; i++) { R[i] = array[right - middle + i]; } // merge two sorted array to one int leftIndex = 0, rightIndex = 0; int k = left; for(leftIndex = 0; leftIndex < n1; leftIndex++) { while(rightIndex < n2) { if (L[leftIndex] < R[rightIndex]) { array[k] = L[leftIndex]; k++; break; } else { array[k] = R[rightIndex]; k++; rightIndex++; } } if (rightIndex == n2 - 1) { break; } } // assign the remaining part while(leftIndex < n1) { array[k++] = L[leftIndex++]; } while (rightIndex < n2) { array[k++] = R[rightIndex++]; } } }