Bismillahir Rahmanir Rahim

While solving the CSES-BOOK Shop problem, I used TOP DOWN approach having complexity O(n.m) approach at first. But it gives TLE. But when I used bottom up approach, it got accepted. Why my Top Down approach got TLE ?I am confused.

TOP DOWN approach

#include<bits/stdc++.h>
#include<iostream>
#define ff first
#define ss second 
#define pb push_back
#define ll long long
#define noob ios::sync_with_stdio(0);cin.tie(0);
using namespace std;
int n;
int a[1005],b[1005],dp[1005][100005];
int solve(int i,int m){
	if(i==n){
		return 0;
	}
	if(dp[i][m]!=-1)return dp[i][m];
	int ans=0;
	ans=max(ans,solve(i+1,m));
	if(m>=a[i]){
		ans=max(ans,solve(i+1,m-a[i])+b[i]);
	}
	return dp[i][m]= ans;
}
int main(){
	int m;
	cin>>n>>m;
	memset(dp,-1,sizeof(dp));
	for(int i=0;i<n;i++)cin>>a[i];
	for(int j=0;j<n;j++)cin>>b[j];
	cout<<solve(0,m)<<endl;
	return 0;
	
    
}

BOTTOM UP approach

#include<bits/stdc++.h>
#include<iostream>
#define ff first
#define ss second 
#define pb push_back
#define ll long long
#define noob ios::sync_with_stdio(0);cin.tie(0);
using namespace std ;
int main(){
    int n,m;
    cin>>n>>m;
    vector<int>a(n),b(n);
    for(int i=0;i<n;i++)cin>>a[i];
    for(int j=0;j<n;j++)cin>>b[j];
    int dp[n+1][m+1];
    memset(dp,0,sizeof(dp));
    for(int i=1;i<=n;i++){
        for(int j=0;j<=m;j++){
            dp[i][j]=dp[i-1][j];
            if(j>=a[i-1]){
                dp[i][j]=max(dp[i][j],dp[i-1][j-a[i-1]]+b[i-1]);
            }
        }
    }
    cout<<dp[n][m]<<endl;
}