[算法 笔记] 求近似和

　　问题出自blog：http://blog.csdn.net/v_july_v/article/details/6803368

　　问题描述：有n个正实数（按照大小升序排列），另一个实数m。需要算出若干个x，使这个几个x的和与m最接近。请描述实现算法啊，并指出算法复杂度。

　　利用贪婪算法，从后向前仅考虑当前计算结果最接近实数m的值，时间复杂度O（n）

　　源代码：

  1 #include <stdio.h>
  2 #include <stdlib.h> // for srand(), rand()
  3 #include <time.h>   // for time()
  4 
  5 int which_sum_is_near_number( const int *array_number,
  6                               const int n_len,
  7                               const int equal_number )
  8 {
  9     int start_index = n_len - 1,
 10         *array_path = NULL,
 11         path_index  = 0,
 12         tmp_summary = 0,    // 计算临时和
 13         min_diff    = array_number[n_len - 1];    // 计算最小差值
 14 
 15     // 检查函数参数
 16     if ( NULL == array_number || n_len <= 0
 17         || equal_number <= 0 )
 18     {
 19         printf( "Error for argument.\n" );
 20         return -1;
 21     }
 22 
 23     // 分配计算路径存储空间
 24     array_path = (int *) calloc( n_len, sizeof(int) );
 25     if ( NULL == array_path )
 26     {
 27         printf( "Out of memory.\n" );
 28         return -1;
 29     }
 30 
 31     // 计算差值
 32     for ( ; start_index >= 0; --start_index )
 33     {
 34         tmp_summary += array_number[start_index];
 35 
 36         // 若计算的临时和大于要求值或计算差值大于当前最小差值，
 37         // 则排除本次计算值
 38         if ( tmp_summary > equal_number
 39             || ( equal_number - tmp_summary ) > min_diff )
 40         {
 41             tmp_summary -= array_number[start_index];
 42             continue;
 43         }
 44 
 45         // 添加路径元素
 46         array_path[path_index++] = array_number[start_index];
 47         // 更新最小差值
 48         min_diff = equal_number - tmp_summary;
 49         if ( tmp_summary == equal_number )
 50         {
 51             break;
 52         }
 53     }
 54 
 55     // 输出计算的路径值
 56     printf( "\nThe elements of path is \n" );
 57     while ( --path_index >= 0 )
 58     {
 59         printf( " %d", array_path[path_index] );
 60     }
 61     printf( "\n" );
 62     printf( "The difference is %d.\n", min_diff );
 63 
 64     // 释放空间
 65     free( array_path );
 66     array_path = NULL;
 67 
 68     return min_diff;
 69 }
 70 
 71 int compare_int( const void *lhs, const void *rhs )
 72 {
 73     return *(int *)lhs - *(int *)rhs;
 74 }
 75 
 76 void test_function()
 77 {
 78     const int array_size = 20;
 79     int array_number[array_size],
 80         start_index = 0,
 81         equal_number = 0;
 82 
 83     srand( (unsigned) time(0) );
 84     for ( ; start_index < array_size; ++start_index )
 85     {
 86         array_number[start_index] = rand() % 100;
 87     }
 88     equal_number = rand() % 200;
 89 
 90     // sorting
 91     qsort( array_number, array_size, sizeof(int), compare_int );
 92 
 93     printf( "\nThe elements in array is \n" );
 94     for ( start_index = 0; start_index < array_size; ++start_index )
 95         printf( " %d", array_number[start_index] );
 96     printf( "\nThe number is %d.\n", equal_number );
 97 
 98     which_sum_is_near_number( array_number, array_size, equal_number );
 99 }
100 
101 int main()
102 {
103     test_function();
104 
105     return 0;
106 }
View Code

　　

　　有问题欢迎大家知道！！

posted @ 2013-08-22 15:31  life91  阅读(226)  评论(0)  编辑  收藏  举报