代码

#include <vector>
#include <algorithm>

void sort(unsigned *a, int n) {
    std::vector<unsigned> sum(256);
    std::vector<unsigned> temp(n);

    for (int j = 0; j < 4; j++) {
        unsigned* src = (j & 1) ? temp.data() : a;
        unsigned* dst = (j & 1) ? a : temp.data();

        for (int i = 0; i < n; i++) {
            sum[(src[i] >> (j * 8)) & 255]++;
        }

        for (int i = 1; i < 256; i++) {
            sum[i] += sum[i - 1];
        }

        for (int i = n - 1; i >= 0; i--) {
            unsigned digit = (src[i] >> (j * 8)) & 255;
            dst[--sum[digit]] = src[i];
        }

        std::fill(sum.begin(), sum.end(), 0);
    }
}

评测结果