题意：给出p个数字，使用+，-，*，/，这四个运算符使得最后结果等于n（四个运算符的优先级相同）

思路：回溯+剪枝，不剪枝的话会TLE。用一个标记数组记录在计算的结果。

#include 
  
   
#include 
   
     #include 
    
      #include 
     
       #include 
      
        using namespace std; const int MAXN = 32000; const int N = 105; int arr[N], vis[N][MAXN * 2 + 5]; int n, m, flag; char path[N]; bool judge(int cur, int sum) { return abs(sum) <= MAXN && !vis[cur][sum + MAXN]; } void dfs(int cur, int sum) { if (flag) return; if (cur == n) { if (sum == m) flag = 1; return; } for (int i = 0; i < 4; i++) { if (i == 0 && judge(cur, sum + arr[cur])) { vis[cur][sum + arr[cur] + MAXN] = 1; path[cur] = '+'; dfs(cur + 1, sum + arr[cur]); } if (i == 1 && judge(cur, sum - arr[cur])) { vis[cur][sum - arr[cur] + MAXN] = 1; path[cur] = '-'; dfs(cur + 1, sum - arr[cur]); } if (i == 2 && judge(cur, sum * arr[cur])) { vis[cur][sum * arr[cur] + MAXN] = 1; path[cur] = '*'; dfs(cur + 1, sum * arr[cur]); } if (i == 3 && arr[cur] != 0 && judge(cur, sum / arr[cur])) { vis[cur][sum / arr[cur] + MAXN] = 1; path[cur] = '/'; dfs(cur + 1, sum / arr[cur]); } if (flag) return; } } void outPut() { for (int i = 0; i < n; i++) { if (i) printf("%c%d", path[i], arr[i]); else printf("%d", arr[i]); } printf("=%d\n", m); } int main() { int cas; scanf("%d", &cas); while (cas--) { scanf("%d", &n); for (int i = 0; i < n; i++) scanf("%d", &arr[i]); scanf("%d", &m); memset(vis, 0, sizeof(vis)); vis[0][arr[0] + MAXN] = 1; flag = 0; dfs(1, arr[0]); if (flag) outPut(); else printf("NO EXPRESSION\n"); } return 0; }