归并排序Merge Sort

//C语言实现

void mergeSort(int array[],int first, int last)
{
    if (first < last)//拆分数列中元素只剩下两个的时候，不再拆分
    {
        int mid = (first + last) / 2;
        //递归拆分数组
        mergeSort(array, first, mid);
        mergeSort(array, mid + 1, last);
        //归并两个数组
        merge(array, first, mid, last);     
    }
}

void merge(int array[], int first,int mid,int last)
{
    int i = first, j = mid + 1, k = first;
    int temp[last + 1];
    
    //从两个数列的第一个开始判断
    while (i <= mid && j <= last)
        if (array[i] <= array[j])
            temp[k ++] = array[i ++];
        else
            temp[k ++] = array[j ++];
    
    //如果有剩余，补充进入数组
    while (i <= mid)    
        temp[k ++] = array[i ++];
    while (j <= last)
        temp[k ++] = array[j ++];
    
    //复制数组
    while (first <= last)
    {
        array[first] = temp[first];
        first ++;
    }
}

//Objective-C实现
//通过NSMutableArray的Category实现

//.h文件
@interface NSMutableArray (ArraySort)

- (void)mergeSort;

@end

//.m文件

#import "NSMutableArray+ArraySort.h"

@implementation NSMutableArray (ArraySort)

- (void)mergeSort
{
    [self sortByStartIndex:0 endIndex:self.count - 1];
}

- (void)sortByStartIndex:(int)start endIndex:(int)end
{
    if (start < end)
    {
        int mid = (start + end) / 2;
        [self sortByStartIndex:start endIndex:mid];
        [self sortByStartIndex:mid + 1 endIndex:end];
        [self mergeByStartIndex:start midIndex:mid endIndex:end];
    }
}

- (void)mergeByStartIndex:(int)start midIndex:(int)mid endIndex:(int)end
{
    int i = start,j = mid + 1;
    NSMutableArray *tempArray = [[NSMutableArray alloc] initWithCapacity:end + 1];
    
    while (i <= mid && j <= end)
        if ([[self objectAtIndex:i] integerValue] <= [[self objectAtIndex:j] integerValue])
            [tempArray addObject:[self objectAtIndex:i ++]];
        else
            [tempArray addObject:[self objectAtIndex:j ++]];
    
    while (i <= mid)
        [tempArray addObject:[self objectAtIndex:i ++]];
    while (j <= end)
        [tempArray addObject:[self objectAtIndex:j ++]];
    
    for (id object in tempArray)
        [self replaceObjectAtIndex:start++ withObject:object];
}

@end