HDU 1015 Safecracker ( DFS )

Safecracker

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)

Total Submission(s): 13280    Accepted Submission(s): 6923

Problem Description

=== Op tech briefing, 2002/11/02 06:42 CST === 
"The item is locked in a Klein safe behind a painting in the second-floor library. Klein safes are extremely rare; most of them, along with Klein and his factory, were destroyed in World War II. Fortunately old Brumbaugh from research knew Klein's secrets and wrote them down before he died. A Klein safe has two distinguishing features: a combination lock that uses letters instead of numbers, and an engraved quotation on the door. A Klein quotation always contains between five and twelve distinct uppercase letters, usually at the beginning of sentences, and mentions one or more numbers. Five of the uppercase letters form the combination that opens the safe. By combining the digits from all the numbers in the appropriate way you get a numeric target. (The details of constructing the target number are classified.) To find the combination you must select five letters v, w, x, y, and z that satisfy the following equation, where each letter is replaced by its ordinal position in the alphabet (A=1, B=2, ..., Z=26). The combination is then vwxyz. If there is more than one solution then the combination is the one that is lexicographically greatest, i.e., the one that would appear last in a dictionary." 

v - w^2 + x^3 - y^4 + z^5 = target 

"For example, given target 1 and letter set ABCDEFGHIJKL, one possible solution is FIECB, since 6 - 9^2 + 5^3 - 3^4 + 2^5 = 1. There are actually several solutions in this case, and the combination turns out to be LKEBA. Klein thought it was safe to encode the combination within the engraving, because it could take months of effort to try all the possibilities even if you knew the secret. But of course computers didn't exist then." 

=== Op tech directive, computer division, 2002/11/02 12:30 CST === 

"Develop a program to find Klein combinations in preparation for field deployment. Use standard test methodology as per departmental regulations. Input consists of one or more lines containing a positive integer target less than twelve million, a space, then at least five and at most twelve distinct uppercase letters. The last line will contain a target of zero and the letters END; this signals the end of the input. For each line output the Klein combination, break ties with lexicographic order, or 'no solution' if there is no correct combination. Use the exact format shown below."

 

Sample Input

1 ABCDEFGHIJKL 11700519 ZAYEXIWOVU 3072997 SOUGHT 1234567 THEQUICKFROG 0 END

 

Sample Output

LKEBA YOXUZ GHOST no solution

 

Source

Mid-Central USA 2002

思路：为了保证按照最大字典序输出，先将字符串中元素按照从大到小排序，然后从前往后搜索（也就是从最大的开始搜索），搜索到满足题意的解后将该解保存起来。

#include 
#include 
#include 

using namespace std;
char s[15], a[15];
int vis[15];
int step, len, n;

bool cmp(const char &a, const char &b)//从大到小排
{
    return a > b;
}

int dfs(int sum, int step)
{
    if (step == 5)
    {
        if (sum == n)
            return 1;
        else 
            return 0;
    }

    int temp;        //定义每个数字前面的符号
    if (step%2 == 0)
        temp = 1;
    else
        temp = -1;

    for (int i=0; i> n >> s)
    {
        if (n==0 && strcmp(s, "END")==0)
            break;
        memset(vis, 0, sizeof(vis));
        len = strlen(s);
        step = 0;
        sort(s, s+len, cmp);//为了保证输出的字典序最大，按照从大到小排序然后从前往后搜索

        if (dfs(0, 0))
            printf("%s\n", a);
        else 
            printf("no solution\n");
    }
    return 0;
}