不用递归编写函数计算从m选n的组合数？

不能用递归函数啊

不用递归就用循环。从m选n的组合数为((m-n+1)/1)((m-n+2)/2)...(m/n)，可以保存p，每次都让p乘(m-n+i)/i，i从1循环到n就得到了结果。注意((m-n+1)/1)...((m-n+i)/i)是从m-n+i选i的组合数，是一个整数，因此我们只需要用整形变量存储p，且得到的是精确值。
以下是样例C++程序：
unsigned comb(unsigned m, unsigned n)
{
unsigned p = 1;
for (unsigned i = 1, j = m - n + 1; i <= n; ++i, ++j) {
p *= j;
p /= i;
}
return p;
}

函数名你自己改吧 非递归： #include &lt;iostream&gt; using namespace std; long c(long m, long n) { long result=1; if (n&gt;m-n) { n=m-n; //c(m,n)==c(m,m-n) 取较小的来计算 } for (int i=1;i&lt;=n;i++) { result*=(long)((double)(m-n+i)/i);// } return result; } int main() { long m,n; while (cin&gt;&gt;n&gt;&gt;m) { cout&lt;&lt;c(m,n)&lt;&lt;endl; } //system("pause"); return 0; } 递归： long c(int m, int n) { if (n==1||n==0) return m; return (double)c(m-1,n-1)*m/n; } int main() { long m,n; while (cin&gt;&gt;m&gt;&gt;n) { cout&lt;&lt;c(m,n)&lt;&lt;endl; } system("pause"); return 0; }

//例：从5个数11 13 15 17 19中选3个数的方法 vc编写 #include<iostream> using namespace std; int main() { int a[5]={11,13,15,17,19}; int i,j,k,l,m,n=0; for(i=0;i<=1;i++) for(j=0;j<=1;j++) for(k=0;k<=1;k++) for(l=0;l<=1;l++) for(m=0;m<=1;m++) if(i+j+k+l+m==3) { if(i==1) cout<<" "<<a[0]<<" "; if(j==1) cout<<" "<<a[1]<<" "; if(k==1) cout<<" "<<a[2]<<" "; if(l==1) cout<<" "<<a[3]<<" "; if(m==1) cout<<" "<<a[4]<<" "; n++; cout<<endl; } cout<<"共有"<<n<<"种选法"<<endl; return 0; }

有人为你做好了，这是我搜集的一个不错的组合算法，需要了解算法可以参考： #ifndef __btb_combination_hpp_def #define __btb_combination_hpp_def #include <algorithm> namespace btb { //////////////////////////////////////////////////////////////////// // combination for STL permutation // // init_combination (first, middle, last) // adjust_combination (first, middle, last) // combination (first1, last1, first2, last2) // next_combination (first, middle, last) // prev_combination (first, middle, last) //////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////// // separate_merge: merge [first1, last1) and [first2, last2), then fill // the min element to [first1, last1), left to [first2, // last2) //////////////////////////////////////////////////////////////////// template <typename Iter> void separate_merge (Iter first1, Iter last1, Iter first2, Iter last2) { typedef typename std::iterator_traits<Iter>::value_type value_type; typedef typename std::iterator_traits<Iter>::difference_type diff_t; if ((first1 == last1) || (first2 == last2)) return; first1 = std::upper_bound(first1, last1, *first2); if (first1 == last1) return; last2 = std::lower_bound(first2, last2, *(last1-1)); if (first2 == last2) return; diff_t len1 = std::distance(first1, last1); diff_t len2 = std::distance(first2, last2); bool min1 = len1 < len2; diff_t lent = min1 ? len1 : len2; value_type *tmp = new value_type[lent]; if (min1) { std::copy(first1, last1, tmp); Iter p = first2; value_type* q = tmp; Iter i; for (i = first1; i != last1; ++i) if (*p < *q) *i = *p++; else *i = *q++; for (i = first2; (i != last2) && (p != last2); ++i) if (*p < *q) *i = *p++; else *i = *q++; for (; i != last2; ++i, ++q) *i = *q; } else { std::copy(first2, last2, tmp); Iter p = last1; value_type* q = tmp+lent; Iter i; for (Iter i = last2; i != first2;) if (*(p-1) < *(q-1)) *--i = *--q; else *--i = *--p; for (i = last1; (i != first1) && (p != first1);) if (*(p-1) < *(q-1)) *--i = *--q; else *--i = *--p; for (; i != first1;) *--i = *--q; } delete[] tmp; } /////////////////////////////////////////////////////////////////// // __combination_sort: merge sort the [first, last) /////////////////////////////////////////////////////////////////// template <typename Iter> void __combination_sort (Iter first, Iter last) { typedef typename std::iterator_traits<Iter>::difference_type diff_t; diff_t len = std::distance(first, last); if (len <= 1) return; if (len == 2) { if (*first > *--last) std::iter_swap (first, last); } else { Iter middle = first; std::advance(middle, len / 2); __combination_sort (first, middle); __combination_sort (middle, last); separate_merge (first, middle, middle, last); } } ////////////////////////////////////////////////////////////////////// // init_combination: init the (first, midle, last) to the min or the // max combination ////////////////////////////////////////////////////////////////////// template <typename Iter> void init_combination (Iter first, Iter middle, Iter last, bool min = true) { __combination_sort (first, middle); __combination_sort (middle, last); if (min) separate_merge (first, middle, middle, last); else separate_merge (middle, last, first, middle); } ////////////////////////////////////////////////////////////////////// // adjust_combination: make the (first, middle, last) to a right // combination. [first, middle) are the elements // selected in, [middle, last) are selected out ////////////////////////////////////////////////////////////////////// template <typename Iter> void adjust_combination (Iter first, Iter middle, Iter last) { __combination_sort (first, middle); __combination_sort (middle, last); } ///////////////////////////////////////////////////////////////////// // combination: get next combination. // // [first1, last1): the elements selected in \\ // [first2, last2): the elements selected out ///////////////////////////////////////////////////////////////////// template <typename Iter> bool combination (Iter first1, Iter last1, Iter first2, Iter last2) { if ((first1 == last1) || (first2 == last2)) return false; Iter qmax = last2; --qmax; Iter pout1 = std::lower_bound(first1, last1, *qmax); bool fin = pout1 == first1; Iter left1, left2; if (!fin) { Iter pout = pout1; --pout; Iter qin = std::upper_bound(first2, last2, *pout); std::iter_swap (pout, qin); left1 = pout; ++left1; left2 = qin; ++left2; } else { left1 = first1; left2 = first2; } separate_merge (left1, last1, left2, last2); return !fin; } ///////////////////////////////////////////////////////////////////// // next_combination: get next combination. // // [first, middle): the elements selected in \\ // [middle, last): the elements selected out ///////////////////////////////////////////////////////////////////// template <typename Iter> inline bool next_combination (Iter first, Iter middle, Iter last) { return combination (first, middle, middle, last); } ///////////////////////////////////////////////////////////////////// // prev_combination: get prev combination. // // [first, middle): the elements selected in \\ // [middle, last): the elements selected out ///////////////////////////////////////////////////////////////////// template <typename Iter> inline bool prev_combination (Iter first, Iter middle, Iter last) { return combination (middle, last, first, middle); } } #endif // 上面是库文件，下面是个测试： #include <iostream> #include <iterator> using namespace std; int main() { int a[] = {1, 2, 3, 4, 5}; int iSize = sizeof(a)/sizeof(a[0]); int* piBeg = a; int* piMid = a + iSize / 2 - (iSize & 1 ? 0 : 1); int* piEnd = a + iSize; // C(3, 5); int iSelect = 3; // 如果数组是无序的首先要初始化，第一，三个参数分是数组的首地址和超尾地址 // 第二个参数是数组中间一个元素的地址 // 该地址应该为: // addr = a + n/2 (n = 2*x+1, x = {1,2,3,...}) // addr = a + n/2-1 (n = 2*x, x = {1,2,3,...}) btb::init_combination(piBeg, piMid, piEnd); do { copy(piBeg, piBeg + iSelect, ostream_iterator<int>(cout, " ")); cout << endl; }while(btb::next_combination(piBeg, piBeg + iSelect, piEnd)); }

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