求N的所有因子（约数）

#include<stdio.h>
#include<math.h>
#include<string.h>
#include<stdlib.h>
#define nmax 100000
int prime[nmax], flag[nmax], factor[nmax], cfactor[nmax], divisor[nmax];
int plen, flen, dlen;
void init() {
    memset(flag, -1, sizeof(flag));
    int i, j;
    for (i = 2, plen = 0; i < nmax; i++) {
        if (flag[i]) {
            prime[plen++] = i;
        }
        for (j = 0; (j < plen) && (i * prime[j] < nmax); j++) {
            flag[i * prime[j]] = 0;
            if (i % prime[j] == 0) {
                break;
            }
        }
    }
}
void findFactor(int n) {
    int i, te, cnt;
    te = (int) (sqrt(n * 1.0));
    for (i = 0, flen = 0; (i < plen) && (prime[i] <= te); i++) {
        if (n % prime[i] == 0) {
            cnt = 0;
            while (n % prime[i] == 0) {
                cnt++;
                n /= prime[i];
            }
            factor[flen] = prime[i];
            cfactor[flen] = cnt;
            flen++;
        }
    }
    if (n > 1) {
        factor[flen] = n;
        cfactor[flen] = 1;
        flen++;
    }
}
void dfs(int k, int now) {
    if (k == flen) {
        divisor[dlen++] = now;
        return;
    }
    int i;
    for (i = 0; i < cfactor[k]; i++) {
        now = now * factor[k];
        dfs(k + 1, now);
    }
    for (i = 0; i < cfactor[k]; i++) {
        now = now / factor[k];
    }
    dfs(k + 1, now);
}
int cmp(const void *a, const void *b) {
    return *(int *) a - *(int *) b;
}
int main() {
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif
    init();
    int n, i;
    while (~scanf("%d", &n)) {
        findFactor(n);
        dlen = 0;
        dfs(0, 1);
        qsort(divisor, dlen, sizeof(divisor[0]), cmp);
        for (i = 0; i < dlen; i++) {
            printf("%d ", divisor[i]);
        }
        printf("\n");
    }
    return 0;
}

另一种方法为：

#define nmax 31625
#define fac 50005
map<int, int> mymap;
int prime[nmax], plen, factor[fac], pfactor[fac], cpfactor[fac], len_pfactor,
        len_factor;
void dfs(int p, int c, long long nn, int n) {
    int i, r;
    long long te;
    for (i = p; i < len_pfactor; i++) {
        if (c > cpfactor[i]) {
            dfs(i + 1, 0, nn, n);
        }
        te = nn * pfactor[i];
        if (te >= n) {
            return;
        }
        r = (int) te;
        if (n % r == 0) {
            if (!mymap[r]) {
                factor[len_factor++] = r;
                mymap[r] = 1;
            }
        }
        dfs(i, c + 1, te, n);
    }
}
        solve(n);
        factor[0] = 1;
        len_factor = 1;
        mymap.clear();
        dfs(0, 0, 1, n);
        if (n != 1) {
            factor[len_factor++] = n;
        }