pku 1270 Following Orders DFS+拓扑排序

题意很清晰.

可以利用dfs遍历每一组值,然后加上剪枝条件.

由于剪枝可以利用当前节点和已遍历节点的顺序关系,可以使用拓扑排序.

 

注意: 输入的第一行不一定是有序的,我在这里sort了一下才过.

 

小结: 拓扑排序用于当序列中的元素有顺序关系时求可行的序列.

 

 

 #include <iostream>#include <algorithm>#include <vector>using namespace std;bool edge[26][26];int inDegree[26];char str[50];char strTemp[250];char ans[27];int cnt;bool visited[26];void dfs(int n){if(n == cnt){for(int i = 0; i < cnt; ++i)printf("%c", ans[i]);printf("/n");return;}for (int i = 0; i < cnt; ++i){if(!visited[str[i]-'a'] && !inDegree[str[i]-'a']){visited[str[i]-'a'] = true;for(int j = 0; j < cnt; ++j)if(edge[str[i]-'a'][str[j]-'a'])inDegree[str[j]-'a']--;ans[n] = str[i];dfs(n+1);visited[str[i]-'a'] = false;for(int j = 0; j < cnt; ++j)if(edge[str[i]-'a'][str[j]-'a'])inDegree[str[j]-'a']++;}}}int main(){while(cin.getline(str, 50)){cnt = (strlen(str)+1)/2;for(int i = 0; i < cnt; ++i)str[i] = str[i*2];str[cnt] = '/0';sort(str, str+cnt);cin.getline(strTemp, 250);memset(edge, 0, sizeof(edge));memset(visited, 0, sizeof(visited));memset(inDegree, 0, sizeof(inDegree));for(int i = 0; i < strlen(strTemp)+1; i+=4){edge[strTemp[i]-'a'][strTemp[i+2]-'a'] = true;inDegree[strTemp[i+2]-'a']++;}dfs(0);printf("/n");}return 0;}