代码

#include <cstring>

unsigned arr[100000000];
unsigned *b = arr, *t;
int z[256];

void sort(unsigned *a, int n) {
  for (int D = 0; D < 4; D++) {
    memset(z, 0, sizeof(z));
    unsigned char *start = (unsigned char *)a;
    unsigned char *end = (unsigned char *)(a + n);
    unsigned char *c = start + D;
    while (c < end) {
      ++z[*c];
      c += 4;
    }
    for (int i = 1; i < 256; i++) {
      z[i] += z[i - 1];
    }
    unsigned *p = (unsigned *)end;
    while (c >= start) {
      c -= 4;
      b[--z[*c]] = *(--p);
    }
    t = a;
    a = b;
    b = t;
  }
}

评测结果