Data structure and algorithm analysis (C language description) Exercise 1.1

Title: Write a program to solve the selection problem. Make k = N/2. Draw a table showing how long your program will run when n is a different value.

(There is a set of n number to determine the largest of the K, called the selection problem (selection problem). ）

Idea: Read the pre-K number to the TEMP array tmp (and sort in descending order). Then read the subsequent number x one by one, and when X is greater than the number of K, add it to the array tmp (and in descending order). Finally returns the value on the position k-1.

Realize:

1#include <stdio.h>2#include <stdlib.h>3#include <time.h>4 5 #defineN 100006 7 int Select(intArr[],intNintk);8 9 intMainvoid)Ten { One     int*arr; A     intvalue; - clock_t elapse; -  theSystem"Color 0A"); -  - Srand ((unsigned) time (NULL)); -Arr = (int*)malloc(sizeof(int) *N); +      for(intj =0; J < N; J + +) -     { +ARR[J] = rand ()%100000; Aprintf"%d", Arr[j]); at     } -Putchar ('\ n'); -  -Elapse =clock (); -Value =Select(arr, n, N/2); -elapse = Clock ()-elapse; inprintf"Value:%d, elapsed:%.4lfs\n", Value, (Double) Elapse/ +); -  to      Free(arr); +System"Pause"); -     return 0; the } *   $ /*Select the k largest in the array*/Panax Notoginseng int Select(intArr[],intNintk) - { the     int*tmp; +     intI, J, ret; A  theTMP = (int*)malloc(sizeof(int) *k); +tmp[0] = arr[0]; -      for(i =1; I < K; i++)//read k elements and sort in descending order $     { $Tmp[i] =Arr[i]; -          for(j = i; j >0; j--) -         { the             if(Arr[i] > Tmp[j-1]) -             {WuyiTMP[J] = tmp[j-1]; theTmp[j-1] =Arr[i]; -             } Wu         } -     } About  $      for(i = k; i < n; i++)//read in arr[k] -     { -         if(Tmp[k-1] <Arr[i]) -         { ATmp[k-1] =Arr[i]; +              for(j = k-1; J >0; j--) the             { -                 if(Arr[i] > Tmp[j-1]) $                 { theTMP[J] = tmp[j-1]; theTmp[j-1] =Arr[i]; the                 } the             } -         } in     } the  theret = tmp[k-1]; About      Free(TMP); the     returnret; the}

Recording:

N Value	Time-consuming (seconds)
10000	0.0820
20000	0.3260
30000	0.7320
40000	1.3080
50000	2.0320
60000	2.9390
70000	3.9990
80000	5.2160
90000	6.6530
100000	8.1610

Small note: The algorithm can give the result in a reasonable time when the input data is small. This algorithm is impractical if the amount of data is too large.

Data structure and algorithm analysis (C language description) Exercise 1.1