Dp does not works but Greedy does , Why?

help
, ,
1

I am solving a question

on spoj which is kind of similar to activity selection problem .
First tried with dp but dp gave me the wrong ans but when i did it greedily and it worked .

But both the approaches are quite similar till dp not worked , Why is it so ?

DP Solution .

bool cmp ( pair <int,int> &a , pair<int,int> &b ) {
    	if (a.second == b.second)
    		return a.first<b.first;
    	return a.second<b.second;
}

void Go() { 
	int n = 0 ;
	cin >> n ;

	vpi v(n) ;
	for ( auto &i : v ) {
		cin >> i.ff >> i.ss ;
	}

	sort(all(v),cmp) ;
	vi dp(n,1) ;

	for ( int i = 1 ; i < n ; i++ ) {
		int s = 0 , e = i-1 ;
		int ans = 0 ;
		while ( s <= e ) {
			int mid = s+(e-s)/2 ;
			if ( v[mid].ss <= v[i].ff ) {
				ans = max ( ans , dp[mid] ) ;
				s = mid+1 ;
			} else {
				e = mid-1 ;
			}
		} 
		dp[i] += ans ;
	}
	cout << *max_element(all(dp)) << Endl ;	
}

Greedy Solution :

bool cmp ( pair <int,int> &a , pair<int,int> &b ) {
    if (a.second == b.second)
        return a.first<b.first;
    return a.second<b.second;
}

void Go() { 
    int n = 0 ;
    cin >> n ;
    vpi v(n) ;
    for ( auto &i : v ) {
        cin >> i.ff >> i.ss ;
    }

    sort(all(v),cmp) ;
    int ans = 0 ;
    int end = -1 ;

    for ( int i = 0 ; i < n ; i++ ) {
        if ( v[i].ff >= end ) {
            ans++ ;
            end = v[i].ss ;
        }
    }
    cout << ans << Endl ;
}

My question is that if we are doing exactly same thing in both apporaches why dp solution does not works ?