搜索 + 剪枝 --- POJ 1101 : Sticks

 Sticks#

Problem's Link:   http://poj.org/problem?id=1011
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Mean: 

http://poj.org/problem?id=1011&lang=zh-CN&change=true

analyse:

爆搜，但是其中蕴含着很多剪枝。

Time complexity: O(n^2)

 

Source code: 

 

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//  Memory   Time //  1347K     0MS //   by : Snarl_jsb //   2014-11-07-17.14 #include<algorithm> #include<cstdio> #include<cstring> #include<cstdlib> #include<iostream> #include<vector> #include<queue> #include<stack> #include<map> #include<string> #include<climits> #include<cmath> #define N 1000010 #define LL long long using namespace std;   int n,sum,len; vector<int> sti(65); vector<bool> used(sti.size());   // k-----从第k根开始往后判断 // total-----该回合还剩下的长度 // sum----未选取的棍子的总长度 bool dfs(int k,int total,int sum) {     if(total==0)     {         sum-=len;         if(sum==0)         {             return true;         }         else         {             total=len;             for(k=0;used[k];++k); //找出未选的最靠前的一根             used[k]=1;             //由于从第k根开始选，第k根必选，从k+1根开始搜索             if(dfs(k+1,total-sti[k],sum))                 return true;             used[k]=0;             sum+=len;         }     }     else     {         for(int i=k;i<n;++i)         {             if(sti[i]==sti[i-1]&&(!used[i-1])&&i>0)                 continue;             if(total>=sti[i]&&(!used[i]))             {                 total-=sti[i];                 used[i]=1;                 if(dfs(i,total,sum))                     return true;                 total+=sti[i];                 used[i]=0;                 if(sti[i]==total)　　//剪枝：该段的长度正好等于需要的长度，但是该方案行不通，直接返回false退出该函数                　　 break;             }         }     }     return false; }   bool cmp(int a,int b) {     return a>b; }   int main() {     ios_base::sync_with_stdio(false);     cin.tie(0); //    freopen("C:\\Users\\ASUS\\Desktop\\cin.cpp","r",stdin); //    freopen("C:\\Users\\ASUS\\Desktop\\cout.cpp","w",stdout);     while(cin>>n,n)     {         sum=0;         int tmp;         sti.clear();         for(int i=0;i<n;++i)         {             used[i]=0;             cin>>tmp;             sum+=tmp;             sti.push_back(tmp);         }         sort(sti.begin(),sti.end(),cmp);         bool flag=false;         for(len=sti.front();len<=sum/2;++len)         {             if(sum%len==0)             {                 if(dfs(0,len,sum))                 {                     cout<<len<<endl;                     flag=1;                     break;                 }             }         }         if(!flag)             cout<<sum<<endl;     }     return 0; } /* 9 5 2 1 5 2 1 5 2 1 4 1 2 3 4 8 45 56 78 456 1231 456456 45 123 */