1 #include<iostream>
2 #include <stdio.h>
3 #include <stdlib.h>
4 #include <algorithm>
5 #include <string>
6 #include <string.h>
7 using namespace std;
8
9 const int nMax = 3000;
10 int A[nMax+1];
11 int B[nMax+1];//用来每次5分法后保存要比较的值在A中的下标
12 int AIndex[nMax+1]; //用来保存A的初始化下标
13
14 //通过插入排序获取中位数下标
15 int InsertSort(int A[], int B[], int start, int end)
16 {
17 if (start == end)
18 {
19 return B[start];
20 }
21
22 for (int i = start+1; i <= end; ++i)
23 {
24 int num = A[B[i]];
25 int j = i-1;
26 for ( ; j >= start; --j)
27 {
28 if (num < A[B[j]])
29 {
30 A[B[j + 1]] = A[B[j]];
31 }
32 else
33 {
34 break;
35 }
36 }
37 A[B[j + 1]] = num;
38 }
39
40 return B[(start + end)/2];
41 }
42
43 //获取中位数的中位数的下标
44 int GetMidMid(int A[], int AIndex[], int k, int n)
45 {
46 if (k == n)
47 {
48 return AIndex[n];
49 }
50
51 int len_s = n - k + 1;
52 //筛选出n/5份的中位数
53 int mod = len_s % 5;
54 int len = len_s / 5 + (mod != 0);
55 for (int i = 1, j = k; i<= len && j <= n-mod; ++i, j+=5)
56 {
57 B[i] = InsertSort(A, AIndex,j, j+4);
58 }
59 if (mod != 0)
60 {
61 B[len] = InsertSort(A, AIndex, n - mod + 1, n);
62 }
63 return GetMidMid(A, B, 1, len);
64 }
65
66 //原址排序
67 int Partition(int A[], int p, int n)
68 {
69 int pivot = A[n];
70 int j = p - 1;
71 for (int i = p; i <= n - 1; ++i)
72 {
73 if (A[i] <= pivot)
74 {
75 j++;
76 swap(A[j], A[i]);
77 }
78 }
79
80 swap(A[j + 1], A[n]);
81 return j + 1;
82 }
83
84 int Select(int A[], int k, int n, int i)
85 {
86 if (k == n)
87 {
88 return A[n];
89 }
90
91 int midValueIndex = GetMidMid(A, AIndex, k, n);
92
93 //将该中位数作为主元(pivot element)
94 //使用一次原址重排
95 int pivot = A[midValueIndex];
96 swap(A[midValueIndex], A[n]);
97 int mid = Partition(A, k, n);
98
99 int t = mid - k + 1;
100 if (i == t)
101 {
102 return A[mid];
103 }
104 else if (i < t)
105 {
106 return Select(A, k, mid-1, i);
107 }
108 else
109 {
110 return Select(A, mid+1, n, i-t);
111 }
112 }
113 int main(int argc, char** argv)
114 {
115 int n = 1111;
116 for (int i = 1; i <= n; ++i)
117 {
118 A[i] = i;
119 AIndex[i] = i;
120 }
121
122 //for (int i = 1; i <= n; ++i)
123 //{
124 // cout << A[i] << " ";
125 //}
126 //cout << endl;
127
128 int equalNum = 0;
129 for (int i = 1; i <= n; ++i)
130 {
131 //随机排列A数组
132 for (int i = 1; i <= n; ++i)
133 {
134 int j = i + rand() % nMax;
135 //swap(A[i], A[j]);
136 A[i] = j;
137 }
138
139 int ans1 = Select(A, 1, n, i);
140 sort(A + 1, A + n + 1);
141 int ans2 = A[i];
142
143 if (ans1 == ans2)
144 {
145 equalNum++;
146 }
147 }
148 cout << n << " " << equalNum << endl;
149 return 0;
150 }
