Rikka with string

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

Total Submission(s): 624    Accepted Submission(s): 243

Problem Description

As we know, Rikka is poor at math. Yuta is worrying about this situation, so he gives Rikka some math tasks to practice. There is one of them:

One day, Yuta got a string which contains n letters but Rikka lost it in accident. Now they want to recover the string. Yuta remembers that the string only contains lowercase letters and it is not a palindrome string. Unfortunately he cannot remember
some letters. Can you help him recover the string?

It is too difficult for Rikka. Can you help her?

Input

This problem has multi test cases (no more than
20).
For each test case, The first line contains a number
n(1≤n≤1000).
The next line contains an n-length string which only contains lowercase letters and ‘?’ – the place which Yuta is not sure.

Output

For each test cases print a n-length string – the string you come up with. In the case where more than one string exists, print the lexicographically first one. In the case where no such string exists,
output “QwQ”.

Sample Input

5
a?bb?
3
aaa

Sample Output

aabba
QwQ

Source

BestCoder Round #37 ($)

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=5202

题目大意：问号处填字母要求字典序最小且非回文

题目分析：遇到问号搜一下，字母从a到z枚举，搜完一组判断是否是回文，不是就退出，否则回溯继续搜

#include <cstdio>
#include <cstring>
char s[1005];
int n;
bool flag;

bool ok()
{
    for(int i = 0; i < n / 2; i++)
        if(s[i] != s[n - i - 1])
            return true;
    return false;
}

void DFS(int pos)
{
    if(pos == n)
    {
        if(ok())
            flag = true;
        return;
    }
    if(s[pos] != '?')
    {
        DFS(pos + 1);
        if(flag)
            return;
    }
    else
    {
        for(char j = 'a'; j <= 'z'; j++)
        {
            s[pos] = j;
            DFS(pos + 1);
            if(flag)
                return;
            s[pos] = '?';
        }
    }
    return;
}

int main()
{
    while(scanf("%d", &n) != EOF)
    {
        flag = false;
        scanf("%s", s);
        DFS(0);
        if(flag)
            printf("%s\n", s);
        else
            printf("QwQ\n");
    }
}