方法是利用快排的想法，效率O(N)
1 /* 2 * use the method of quicksort. The time efficiency is 3 * O(N). 4 * 5 * */ 6 #include <iostream> 7 #include <cstdlib> 8 #include <cstdio> 9 #include <iomanip> 10 11 using namespace std; 12 13 class Array 14 { 15 public: 16 friend istream & operator >> (istream & input, Array & a); 17 friend ostream & operator << (ostream & outpu, Array & a); 18 int operator [] (int p); 19 void exchange(int &s, int &t); 20 int partition(int *p, int l, int r); 21 int k_element(int *p, int l, int r, int k); 22 int getlen(){return len;} 23 private: 24 int *a; 25 int len; 26 }; 27 28 int Array::operator [] (int p) 29 { 30 return k_element(a, 0, len-1, p); 31 } 32 33 int Array::k_element(int *p, int l, int r, int k) 34 { 35 if (l < r) 36 { 37 int q = partition(p, l, r); 38 if (q + 1 == k) 39 return p[q]; 40 else if (q + 1 > k) 41 return k_element(p, l, q-1, k); 42 else 43 return k_element(p, q+1, r, k); //这个地方尤其要注意，应该还是k，而不是k-q-1！ 44 } 45 else return p[l]; 46 } 47 48 ostream & operator << (ostream & output, Array & array) 49 { 50 for (int i = 0; i < array.len; ++i) 51 output << setw(3) << array.a[i]; 52 output << endl; 53 return output; 54 } 55 56 istream & operator >> (istream & input, Array & array) 57 { 58 cout << "intput the length of Array: "; 59 input >> array.len; 60 array.a = new int[array.len+1]; 61 for (int i = 0; i < array.len; ++i) 62 input >> array.a[i]; 63 return input; 64 } 65 66 void Array::exchange(int &s, int &t) 67 { 68 int temp; 69 temp = s; s = t; t = temp; 70 } 71 72 int Array::partition(int *p, int l, int r) 73 { 74 int i = (l+r)/2, j; 75 exchange(p[i], p[r]); 76 int store = l; 77 for (j = l; j < r; ++j) 78 { 79 if (p[j] >= p[r]) 80 { 81 exchange(p[j], p[store]); 82 store++; 83 } 84 } 85 exchange(p[r], p[store]); 86 return store; 87 } 88 89 int main(void) 90 { 91 Array array; 92 93 freopen("in", "r", stdin); 94 95 cin >> array; 96 cout << array; 97 for (int i = 0; i < array.getlen(); ++i) 98 cout << i+1 << "-th: " << array[i+1] << endl; 99 cout << endl; 100 101 return 0; 102 }
写的过程中还是出现了一些错误，以后要认真，尤其是细节，不能想当然，要仔细一点。
