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) / ;
         //递归拆分数组
         mergeSort(array, first, mid);
         mergeSort(array, mid + , 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 + , k = first;
     int temp[last + ];

     //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: endIndex:self.count - ];
 }

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

 - (void)mergeByStartIndex:(int)start midIndex:(int)mid endIndex:(int)end
 {
     int i = start,j = mid + ;
     NSMutableArray *tempArray = [[NSMutableArray alloc] initWithCapacity:end + ];

     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