排序算法之归并排序

归并排序算法的思路是将数组分为两半，对每部分递归地应用归并排序。在两部分都排好序后，对它们进行归并。

public class MergeSort {
	
	public static void sort(int[] list) {
		int len = list.length;
		if(len > 1) {
			int[] firstHalf = new int[len/2];
			System.arraycopy(list, 0, firstHalf, 0, len/2);
			sort(firstHalf);//对list数组前半部分递归调用
			
			int[] lastHalf = new int[len-len/2];
			System.arraycopy(list, len/2, lastHalf, 0, len - len/2);
			sort(lastHalf);//对list数组后半部分递归调用
			
			merge(firstHalf, lastHalf, list);//将排序后的list合并
		}
	}
	
	public static void merge(int[] firstHalf, int[] lastHalf, int[] list) {
		int currentFirst = 0;   //firstHalf当前遍历位置的索引 
		int currentLast = 0;    //firstHalf当前遍历位置的索引 
		int currentList = 0;	//list当前遍历位置的索引 
		
		//每次取出firstHalf和lastHalf中未放入list中的最小数字，将较小的放入list
		while(currentFirst < firstHalf.length && currentLast < lastHalf.length) {
			if(firstHalf[currentFirst] < lastHalf[currentLast]) {
				list[currentList++] = firstHalf[currentFirst++];
			}else {
				list[currentList++] = lastHalf[currentLast++];
			}
		}
		//若lastHalf已全部放入list，将firstHalf剩余的依次放入list
		while(currentFirst < firstHalf.length) {
			list[currentList++] = firstHalf[currentFirst++];
		}
		//若firstHalf已全部放入list，将lastHalf剩余的依次放入list
		while(currentLast < lastHalf.length) {
			list[currentList++] = lastHalf[currentLast++];
		}
	}

	public static void main(String[] args) {
		int[] num = {2,4,6,5,3,7,8,1};
		sort(num);
		for(int i : num) {
			System.out.print(i+" ");
		}
	}

}