swust oj 715--字典序问题（组合数预处理）

 1 #include <iostream>
 2 #include <cstring>
 3 using namespace std;
 4 
 5 char s[7];
 6 int cur[27][27] = { 1 };
 7 
 8 //预处理 利用杨辉三角计算组合数
 9 void init(){
10     int i, left, right;
11     for (i = 1; i <= 26; i++){
12         cur[i][0] = cur[i][i] = 1;
13         left = 1, right = i - 1;
14         while (left <= right){
15             cur[i][left] = cur[i - 1][left - 1] + cur[i - 1][left];
16             cur[i][right--] = cur[i][left++];//组合数性质cur[i][j]=cur[i][i-j];
17         }
18     }
19 }
20 
21 //长度小于len的串的个数
22 int minlen_num(int len){
23     int i, cnt = 0;
24     for (i = 1; i < len; i++)
25         cnt += cur[26][i];
26     return cnt;
27 }
28 
29 //当前长度下当前串前面的个数
30 int enquallen_num(int len){
31     int i, j, cnt = 0, pre = -1, tmp;
32     for (i = 0; i < len; i++){
33         tmp = s[i] - ‘a‘;
34         for (j = pre + 1; j < tmp; j++)
35             cnt += cur[26 - j - 1][len - i - 1];
36         pre = tmp;
37     }
38     return cnt;
39 }
40 
41 bool judge(int len){
42     for (int i = 1; i < len; i++)
43     if (s[i] <= s[i - 1])
44         return false;
45     return true;
46 }
47 
48 int main(){
49     init();
50     int t, cnt, len;
51     cin >> t;
52     while (t--){
53         cnt = 0;
54         cin >> s;
55         len = strlen(s);
56         if (!judge(len)) cout << 0 << endl;
57         else{
58             cnt += minlen_num(len);
59             cnt += enquallen_num(len);
60             cout << cnt + 1 << endl;
61         }
62     }
63     return 0;
64 }