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题目链接：hdu 4961 Boring Sum
题目大意：给定ai数组;
- 构造bi, k=max(j|0
构造ci, k=min(j|i
求∑i=1nbi?ci
解题思路：因为ai≤105,所以预先处理好每个数的因子，然后在处理bi，ci数组的时候，每次遍历一个数，就将其所有的因子更新，对于bi维护最大值，对于ci维护最小值。
#include
#include
#include
#include
using namespace std; typedef long long ll; const int maxn = 1e5; const int INF = 0x3f3f3f3f; int n, arr[maxn+5], b[maxn+5], c[maxn+5], v[maxn+5]; vector
g[maxn+5]; void get_factor (int n) { for (int i = 1; i <= n; i++) g[i].clear(); for (int i = 1; i <= n; i++) { for (int j = i; j <= n; j += i) g[j].push_back(i); } } void init () { memset(v, 0, sizeof(v)); for (int i = 1; i <= n; i++) { int u = arr[i]; int k = (v[u] == 0 ? i :v[u]); b[i] = arr[k]; for (int j = 0; j < g[u].size(); j++) v[g[u][j]] = max(v[g[u][j]], i); } memset(v, INF, sizeof(v)); for (int i = n; i >= 1; i--) { int u = arr[i]; int k = (v[u] == INF ? i : v[u]); c[i] = arr[k]; for (int j = 0; j < g[u].size(); j++) v[g[u][j]] = min(v[g[u][j]], i); } } int main () { get_factor(maxn); while (scanf("%d", &n) == 1 && n) { for (int i = 1; i <= n; i++) scanf("%d", &arr[i]); init(); ll ans = 0; for (int i = 1; i <= n; i++) ans = ans + b[i] * 1LL * c[i]; printf("%I64d\n", ans); } return 0; }
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