题目描述
把 1~9 这9个数字分成两组，中间插入乘号，
有的时候，它们的乘积也只包含1~9这9个数字，而且每个数字只出现1次。
比如：
984672 * 351 = 345619872
98751 * 3462 = 341875962
9 * 87146325 = 784316925
…
符合这种规律的算式还有很多，请你计算在所有这些算式中，乘积最大是多少？
输出
输出一个整数表示答案

代码如下：

#include <iostream>
#include <cstring>
using namespace std;
typedef long long LL;
LL maxv = -1;
const int N = 12;
bool vis[N];
bool st[N];bool check(int suma, int sumc) {LL b = (LL)suma * sumc;memset(vis, 0, sizeof(vis));//标记数组记得重置if (!b) {return false;}while (b) {if (vis[b % 10] || b % 10 == 0)return false;vis[b % 10] = true;b = b / 10;}for (int i = 1; i <= 9; i++) {if (!vis[i])return false;}return true;
}void dfs_c(int u, int suma, int sumc) {//if (u > 9) {if (u >= 9) { //边界不要弄错if (check(suma, sumc)) {maxv = max((LL)suma * sumc, maxv);return ;}return ;}for (int i = 1; i <= 9; i++) {if (!st[i]) {st[i] = true;dfs_c(u + 1, suma, sumc * 10 + i);st[i] = false;}}
}void dfs_a(int u, int suma) {if (u) {dfs_c(u, suma, 0);}for (int i = 1; i <= 9; i++) {if (!st[i]) {st[i] = true;dfs_a(u + 1, suma * 10 + i);st[i] = false;}}
}int main() {dfs_a(0, 0);cout << maxv << endl;return 0;
}

#include <iostream>
using namespace std;
typedef long long LL;
const int N = 12;
bool vis[N];
LL maxv = -1;bool check_1(int suma,int sumc)
{int  st[N];memset(st,0,sizeof(st));//必不可少
//	for (int i = 1;i<=9;i++)或者写成这样，反正无论怎么写，都要初始化为0
//	{
//		st[i] = 0
//	}LL sumb = suma*(LL)sumc;while(sumb){st[sumb%10]++;sumb = sumb/10;}for (int i = 1;i<=9;i++){if (st[i]!=1)return false;}return true;
}void dfs_c(int u,int suma,int sumc)
{if (u > 9) return ;if (u==9){if (check_1(suma,sumc) && sumc!=0){LL sumb = (LL)suma*sumc;maxv = max(maxv,sumb);return ;}}for (int i = 1;i<=9;i++){if (!vis[i]){vis[i] = true;dfs_c(u+1,suma,sumc*10+i);vis[i] = false;}}
}void dfs_a(int u,int suma)
{dfs_c(u,suma,0);for (int i = 1;i<=9;i++){if (!vis[i]){vis[i] = true;dfs_a(u+1,suma*10+i);vis[i] = false;}}
}int main()
{dfs_a(0,0);cout<<maxv<<endl;return 0;
}