单词拼接（dfs/回溯/递归）

单词拼接传送门

//单词拼接 
#include<stdio.h>
#include<string.h>
#include<algorithm>
using namespace std;
struct node
{
    char s[31];
    int first, last;
};

node a[1001];
int degree_in[1001], degree_out[1001], m, order[1001];
bool used[1001];

int f()
{
    int x1, x2, ans = 0, i;
    x1 = x2 = 0;
    for (i = 0; i < 26; ++i)
    {
        if (abs(degree_in[i] - degree_out[i]) >= 2)
            return -1;
        else if (degree_in[i] - degree_out[i] == 1)
            x1++;
        else if (degree_in[i] - degree_out[i] == -1)
        {
            x2++;
            ans = i;
        }
    }
    if (x1>1 || x2 > 1) //当时三个度时，必定是 12 和21，相同的不能大于等于2，不然不能构成欧拉回路
        return -1;
    else if (x1 == 0)
    {
        for (i = 0; i < 26; ++i)
            if (degree_out[i])
                return i; //找到一个就行
    }
    else
        return ans;
    
    }
bool cmp(node a, node b)
{
    return strcmp(a.s, b.s) < 0;
}

bool dfs(int st, int cnt)
{
    int i;
    if (cnt == m)
        return 1;
    for (i = 0; i < m; ++i)
    {
        if (a[i].first < st || used[i])
            continue;
        else if (a[i].first > st)
            return false;
        used[i] = true;
        order[cnt] = i;
        if (dfs(a[i].last, cnt + 1))
            return 1;
        used[i] = false;//回溯判断是否形成欧拉路径
    }
    return false;
}


int main()
{
    int N, len, i, start;
    scanf("%d", &N);
    while (N--)
    {
        memset(used, false, sizeof(used));
        memset(degree_out, 0, sizeof(degree_out));
        memset(degree_in, 0, sizeof(degree_in));
        scanf("%d", &m);
        for (i = 0; i < m; ++i)
        {
            scanf("%s", a[i].s);
            len = strlen(a[i].s);
            a[i].first = a[i].s[0] - 'a';
            a[i].last = a[i].s[len - 1] - 'a';
            degree_out[a[i].s[0] - 'a']++;
            degree_in[a[i].s[len - 1] - 'a']++;//注意这里的入肚出度
        }
        start = f();
        if (start == -1)
        {
            printf("***
");
            continue;
        }
        sort(a, a + m, cmp);
        if (!dfs(start, 0))
        {
            printf("***
");
            continue;
        }
        printf("%s", a[order[0]].s);
        for (i = 1; i < m; i++)
            printf(".%s", a[order[i]].s);
        printf("
");
    }
    return 0;
}