Given an unweighted and undirected tree,the task is to find the length of the longest path (from one node to another) in that tree.The length of a path in this case is number of edges we traverse from source to destination.
Example-
Number of nodes=3
Number of edges=2
Edges-
1 2
2 3
Output-
2
(1->2->3 is the longest path in the given tree which has a length of 2 units).
Algorithm-
1.Loop through the vertices, starting a new depth first search whenever the loop reaches a vertex that has not already been included in previous DFS calls.
2.A dist[] array is constructed to record the distances of all the vertices from the starting vertex ie. vertex on which DFS is called.
3.Maximum of all the values of the dist[] array is found and the respective vertex number is found.Let it be v.
4.Now,DFS is called on v and dist[] array records the distances of all the vertices from the vertex v.
5.Maximum of all the values of the dist[] array is the final answer that is it is the length of the longest path in the tree.
Code-
[cpp]
#include<stdio.h>
#include<vector>
#include<algorithm>
#include<iostream>
using namespace std;
vector<int>arr[10005];
int n,a,b,j=0,m;
int color[10005],dist[10005];
//d denotes the distance of the node on which DFS is called from the starting vertex.
void dfs(int node,int d)
{
color[node]=2;
//marking the visited vertices
dist[node]=d;
for(int i=0;i<arr[node].size();++i)
{
if(color[arr[node][i]]==0)
{
dfs(arr[node][i],d+1);
}
}
}
int main()
{
int p;
int i1;
p=1;j=0;
//n is the number of vertices
scanf("%d",&n);
for(i=0;i<n;i++)
arr[i].clear();
for(i=0;i<n;++i)
{
color[i]=0;dist[i]=0;
}
//tree has n-1 edges
while(j<n-1)
{
scanf("%d %d",&a,&b);
//a and b denote the starting and ending vertices of an edge
arr[a-1].push_back(b-1);
arr[b-1].push_back(a-1);
j++;
}
for(i=0;i<n;++i)
{
if(color[i]==0)
dfs(i,0);
}
int max=0;
// p denotes the vertex with maximum distance
for(i=0;i<n;i++)
{
if(dist[i]>=max)
{ max=dist[i];
p=i;
}
}
//resetting the arrays to call DFS again
for(i=0;i<n;++i)
{
color[i]=0;dist[i]=0;
}
//calling dfs on vertex which has max distance
dfs(p,0);
max=0;
for(i=0;i<n;i++)
{
if(dist[i]>=max)
{ max=dist[i];
p=i;
}
}
cout<<max<<endl;
return 0;
}
[/cpp]
