ACM学习历程——HDU5202 Rikka with string（dfs，回文字符串）

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 ($)

 

题目意思就是在？处填入小写字母，要求输出字典序最小的非回文字符串，否则输出QwQ。

这题用dfs对？处进行搜索就行。不过好久没写搜索，当时忘记回溯了，一直报WA。

 

代码：

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <set>
#include <map>
#include <vector>
#include <queue>
#include <string>
#define inf 0xfffffff
#define eps 1e-10

using namespace std;

int n;
char str[1005];
bool flag;

void Input()
{
    flag = false;
    getchar();
    for (int i = 0; i <= n; ++i)
    {
        str[i] = getchar();
    }
}

void Output()
{
    int i = 0;
    while (str[i] != '
')
    {
        printf("%c", str[i]);
        i++;
    }
    printf("
");
}

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

void dfs(int i)
{
    if (flag)
        return;
    while (str[i] != '?' && str[i] != '
')
        i++;
    if (str[i] == '
')
    {
        if (!Cheak())
        {
            flag = true;
        }
        return;
    }
    if (str[i] == '?')
    {
        for (char j = 'a'; j <= 'z'; ++j)
        {
            str[i] = j;
            dfs(i+1);
            if (flag)
                return;
            str[i] = '?';
        }
    }
}

int main()
{
    //freopen("test.txt", "r", stdin);
    while (scanf("%d", &n) != EOF)
    {
        Input();
        dfs(0);
        if (flag)
            Output();
        else
            printf("QwQ
");
    }
    return 0;
}