hdu 1518(dfs)
题目链接 :http://acm.hdu.edu.cn/showproblem.php?pid=1518
Square
Problem Description
Given a set of sticks of various lengths, is it possible to join them end-to-end to form a square?
Input
The first line of input contains N, the number of test cases. Each test case begins with an integer 4 <= M <= 20, the number of sticks. M integers follow; each gives the length of a stick - an integer between 1 and 10,000.
Output
For each case, output a line containing "yes" if is is possible to form a square; otherwise output "no".
Sample Input
3 4 1 1 1 1 5 10 20 30 40 50 8 1 7 2 6 4 4 3 5
Sample Output
yes no yes
题意 :给你n条边,让你用这些边组成正方形(不多不少仅n条)。
分析 :主要是超时问题,注意优化。
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 #include <cstdio> 2 #include <cstring> 3 #include <algorithm> 4 using namespace std; 5 6 int a[22],vis[22]; 7 int n,m,length; //length表示要组成的正方形的边长 8 int ans,flag; 9 10 void dfs(int cnt,int sum, int k) //cnt记录边数,sum记录当前边长,k记录位置 11 { 12 if (cnt == 3) //如果有三条边满足要求,那么第四条边一定满足要求 13 { 14 flag = 1; 15 return ; 16 } 17 if (sum == length) //找到满足要求的边,边数加一,初始化 18 { 19 cnt++; 20 k = 0; 21 sum = 0; 22 } 23 for (int i=k; i<m; i++) 24 { 25 if (!vis[i] && sum + a[i] <= length) 26 { 27 vis[i] = 1; 28 dfs(cnt, sum+a[i], i+1); 29 if (flag) //优化时间,(当找到所有边之后就一直返回,不需要再把之后的代码运行一遍) 30 { 31 return ; 32 } 33 vis[i] = 0; 34 } 35 } 36 } 37 38 int main () 39 { 40 scanf ("%d",&n); 41 while (n--) 42 { 43 ans = 0; 44 scanf ("%d",&m); 45 for (int i=0; i<m; i++) 46 { 47 scanf ("%d",&a[i]); 48 ans += a[i]; 49 } 50 if (ans % 4) //如果所有边长和不是4的倍数,怎样都不能组成正方形 51 { 52 printf ("no\n"); 53 continue ; 54 } 55 memset(vis, 0, sizeof(vis)); 56 flag = 0; 57 length = ans / 4; //记录正方形边长 58 dfs(0, 0, 0); 59 if (flag) 60 printf ("yes\n"); 61 else 62 printf ("no\n"); 63 } 64 return 0; 65 }