STL 源碼剖析 演算法 stl_algo.h -- nth_element

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nth_element
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描述：重新排序，使得[nth,last)內沒有任何一個元素小於[first,nth)內的元素，
但對於[first,nth)和[nth,last)兩個子區間內的元素次序則無任何保證。
思路：
1.以 median-of-3-partition 將整個序列分割為更小的左、右子序列
2.如果 nth 迭代器落於左序列，就再對左子序列進行分割，否則就再對右子序列進行分割
3.直到分割後的子序列長大於3，對最後這個待分割的子序列做 Insertion Sort
圖6-17
複雜度：O(n)
源碼：

template <class RandomAccessIterator>inline void nth_element(RandomAccessIterator first, RandomAccessIterator nth,                        RandomAccessIterator last) {  __nth_element(first, nth, last, value_type(first));}template <class RandomAccessIterator, class T>void __nth_element(RandomAccessIterator first, RandomAccessIterator nth,                   RandomAccessIterator last, T*) {  while (last - first > 3) {    //採用 median-of-3-partition 。參數：(first,last,pivot)//返回一個迭代器，指向分割後的右段第一個元素RandomAccessIterator cut = __unguarded_partition      (first, last, T(__median(*first, *(first + (last - first)/2),                               *(last - 1))));    if (cut <= nth) //如果  nth 落於右段，再對右段實施分割      first = cut;    else  //如果 nth 落於左段，對左段實施分割      last = cut;  }  __insertion_sort(first, last); //對分割後的子序列做 Insertion Sort}template <class RandomAccessIterator, class T>RandomAccessIterator __unguarded_partition(RandomAccessIterator first,                                            RandomAccessIterator last,                                            T pivot) {  while (true) {    while (*first < pivot) ++first;    --last;    while (pivot < *last) --last;    if (!(first < last)) return first;    iter_swap(first, last);    ++first;  }}    

樣本：

int A[] = {7, 2, 6, 11, 9, 3, 12, 10, 8, 4, 1, 5};const int N = sizeof(A) / sizeof(int);nth_element(A, A + 6, A + N);copy(A, A + N, ostream_iterator<int>(cout, " "));// The printed result is "5 2 6 1 4 3 7 8 9 10 11 12".