POJ3278——Catch That Cow

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Catch That Cow

Time Limit: 2000MS		Memory Limit: 65536K
Total Submissions: 114140		Accepted: 35715

Description

Farmer John has been informed of the location of a fugitive cow and wants to catch her immediately. He starts at a point N (0 ≤ N ≤ 100,000) on a number line and the cow is at a point K (0 ≤ K ≤ 100,000) on the same number line. Farmer John has two modes of transportation: walking and teleporting.

* Walking: FJ can move from any point X to the points X - 1 or X + 1 in a single minute
* Teleporting: FJ can move from any point X to the point 2 × X in a single minute.

If the cow, unaware of its pursuit, does not move at all, how long does it take for Farmer John to retrieve it?

Input

Line 1: Two space-separated integers: N and K

Output

Line 1: The least amount of time, in minutes, it takes for Farmer John to catch the fugitive cow.

Sample Input

5 17

Sample Output

4

Hint

The fastest way for Farmer John to reach the fugitive cow is to move along the following path: 5-10-9-18-17, which takes 4 minutes.

Source

USACO 2007 Open Silver

 

终于A掉这道题，BFS新认知。

 

#include<iostream>
#include<cstdio>
#include<queue>
#include<algorithm>
#include<vector>
#include<cmath>
#include<cstring>
#include<string> 

#define N 100010

using namespace std;

void in(int &x){
    register char c=getchar();x=0;int f=1;
    while(!isdigit(c)){if(c=='-') f=-1;c=getchar();}
    while(isdigit(c)){x=x*10+c-'0';c=getchar();}
    x*=f;
}

int a,b,ans;
struct Step{
	int v,w;//位置，步数 
	Step(int xx,int s):v(xx),w(s){ }
};
bool vis[N];
queue<Step>Q;

void BFS(){
	memset(vis,0,sizeof(vis));
	vis[a]=1;Q.push(Step(a,0));
	while(!Q.empty()){
		Step s=Q.front();Q.pop();
		if(s.v==b) {
			ans=s.w;break;
		}
		else {
			if(s.v-1>=0 && !vis[s.v-1]){
				Q.push(Step(s.v-1,s.w+1));
				vis[s.v-1]=1;
			}if(s.v+1<=N &&!vis[s.v+1]){
				Q.push(Step(s.v+1,s.w+1));
				vis[s.v+1]=1;
			}if(s.v*2<=N && !vis[s.v*2]){
				Q.push(Step(s.v*2,s.w+1));
				vis[s.v*2]=1;
			}
		}
	}
}

int main()
{
	in(a);in(b);
	BFS();
	printf("%d
",ans);
	return 0;
}