快排

今天才学习了快排，参考了《算法导论》。

对于n个数的输入数组，最坏的情况运行时间是O（n^2），期望的运行时间为O(nlogn)。

用C实现了下。继续努力。。。

 

#include <stdio.h>
#include <string.h>

int partition ( int *a, int p, int r ) {
    int x = a[r];
    int i = p - 1;
    int tmp;
    for ( int j = p; j <= r - 1; ++j )
         if ( a[j] <= x ) {
              i = i + 1;
              tmp = a[i];
              a[i] = a[j];
              a[j] = tmp;
         }
    tmp = a[i + 1], a[i + 1] = a[r], a[r] = tmp;
    return i + 1;
}


void Qsort ( int *a, int p, int r ) {
     int q;
     if ( p < r ) {
          q = partition ( a, p, r );
          Qsort ( a, p, q - 1 );
          Qsort ( a, q + 1, r );
     }
}

int main ( ) {
    int n;
    while ( scanf ( "%d", &n ) != EOF ) {
          int a[n];
          for ( int i = 1; i <= n; ++i )
               scanf ( "%d", &a[i] );
          Qsort ( a, 1, n );
          for ( int i = 1; i <= n; ++i )
               printf ( "%d ", a[i] );
          printf ( "\n" );
    }
}