G - Longest Path

1

I am getting TLE in this problem .
i am using F(source) = 1 + max(F(neighbours(n1,n2,n3))

Can anyone suggest me better approach.

function :

#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;

ll dp[100001];

ll findLongestPath(vector graph[],ll src){
if(dp[src]!=-1)
return dp[src];

ll result = INT_MIN;
bool isTrue = false;
for(ll neighbour : graph[src]){
    isTrue = true;
    result = max(result,findLongestPath(graph,neighbour) + 1);
}

return (isTrue)?result:0;

}

int main()
{
ios_base:: sync_with_stdio(false);
cin.tie(NULL);

ll v,e;    cin>>v>>e;
memset(dp,-1,sizeof(dp));
vector<ll> graph[100001];

while(e--){
    int a,b;    cin>>a>>b;
    graph[a].push_back(b);
}



ll result = INT_MIN;
for(ll i = 1; i<=v;i++){
    result = max(findLongestPath(graph,i),result);
}

cout<<result<<endl;

}