nth_element 测试程序

 /********************************************************************
 created:    2014/04/29 11:35
 filename:    nth_element.cpp
 author:        Justme0 (http://blog.csdn.net/justme0)

 purpose:    nth_element
 *********************************************************************/

 #include <cstdio>
 #include <cstdlib>
 #include <cstring>

 typedef int Type;

 template <class T>
 inline T * copy_backward(const T *first, const T *last, T *result) {
     const ptrdiff_t num = last - first;
     memmove(result - num, first, sizeof(T) * num);
     return result - num;
 }

 /*
 ** 将 value 插到 last 前面（不包括 last）的区间
 ** 此函数保证不会越界（主调函数已判断），因此以 unguarded_ 开头
 */
 template <class RandomAccessIterator, class T>
 void unguarded_linear_insert(RandomAccessIterator last, T value) {
     RandomAccessIterator next = last;
     --next;
     while(value < *next) {
         *last = *next;
         last = next;
         --next;
     }
     *last = value;
 }

 /*
 ** 将 last 处的元素插到[first, last)的有序区间
 */
 template <class RandomAccessIterator>
 void linear_insert(RandomAccessIterator first, RandomAccessIterator last) {
     Type value = *last;
     if (value < *first) { // 若尾比头小，就将整个区间一次性向后移动一个位置
         copy_backward(first, last, last + );
         *first = value;
     } else {
         unguarded_linear_insert(last, value);
     }
 }

 template <class RandomAccessIterator>
 void insertion_sort(RandomAccessIterator first, RandomAccessIterator last) {
     if (first == last) {
         return ;
     }

     for (RandomAccessIterator ite = first + ; ite != last; ++ite) {
         linear_insert(first, ite);
     }
 }

 template <class T>
 inline const T & median(const T &a, const T &b, const T&c) {
     if (a < b) {
         if (b < c) {
             return b;
         } else if (a < c) {
             return c;
         } else {
             return a;
         }
     } else if (a < c) {
         return a;
     } else if (b < c) {
         return c;
     } else {
         return b;
     }
 }

 template <class ForwardIterator1, class ForwardIterator2>
 inline void iter_swap(ForwardIterator1 a, ForwardIterator2 b) {
     Type tmp = *a;        // 源码中的 T 由迭代器的 traits 得来，这里简化了
     *a = *b;
     *b = tmp;
 }

 /*
 ** 设返回值为 mid，则[first, mid)中迭代器指向的值小于等于 pivot;
 ** [mid, last)中迭代器指向的值大于等于 pivot
 ** 这是 STL 内置的算法，会用于 nth_element, sort 中
 ** 笔者很困惑为什么不用 partition
 */
 template <class RandomAccessIterator, class T>
 RandomAccessIterator unguarded_partition(RandomAccessIterator first, RandomAccessIterator last, T pivot) {
     while(true) {
         while (*first < pivot) {
             ++first;
         }
         --last;
         while (pivot < *last) {    // 若 std::partition 的 pred 是 IsLess(pivot)，这里将是小于等于
             --last;
         }
         if (!(first < last)) {    // 小于操作只适用于 random access iterator
             return first;
         }
         iter_swap(first, last);
         ++first;
     }
 }

 template <class RandomAccessIterator>
 void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last) {
     while (last - first > ) {
         RandomAccessIterator cut = unguarded_partition(first, last, Type(median(
             *first,
             *(first + (last - first) / ),
             *(last - ))));
         if (cut <= nth) {
             first = cut;
         } else {
             last = cut;
         }
     }
     insertion_sort(first, last);
 }

 int main(int argc, char **argv) {
     int arr[] = {, , , , , , , , , , };
     int size = sizeof arr / sizeof *arr;

     nth_element(arr, arr + , arr + size);

     for (int i = ; i < size; ++i) {
         printf("%d ", arr[i]);    // 20 12 22 17 17 22 23 30 30 33 40
     }
     printf("\n");

     system("PAUSE");
     return ;
 }