uva 11728 - Alternate Task(数论)

题目链接：uva 11728 - Alternate Task

题目大意：给出S，求N，要求N所有的因子和为S。

解题思路：枚举因子i，所有整除i的数和加上i。

#include <cstdio>#include <cstring>const int N = 1005;int n, c[N], v[N];void init () {    memset(c, 0, sizeof(c));    memset(v, -1, sizeof(v));    for (int i = 1; i < N; i++) {        for (int j = i; j < N; j += i)            c[j] += i;        if (c[i] < N)            v[c[i]] = i;    }}int main () {    init();    int cas  = 1;    while (scanf("%d", &n) == 1 && n) {        printf("Case %d: %d\n", cas++, v[n]);    }    return 0;}