Following are two codes of same problem SPOJ MIXTURES. The first is getting accepted but the second one is showing time limit exceeded. Although, Both codes are completely same. but the only difference between following codes is in line no. 12 (numbered) i.e int &r in first code and only int r in second. Please tell why the second code getting time limit exceeded and first one is getting accepted.

FIRST CODE

 #include <bits/stdc++.h>

using namespace std;

int a[100],sum[100][100],dp[100][100];
int solve(int ai,int bi){
    if(ai==bi){
       return 0;
    }
    if((ai+1)==bi)
        return a[ai] * a[bi];
12.         **int &r= dp[ai][bi],c;**
        if(r==-1){
            for(int i=ai;i+1<=bi;i++){
                c= sum[ai][i] * sum[i+1][bi] + solve(ai,i) + solve(i+1,bi);
            if(r==-1 || c<r)r=c;
            }
        }
        return r;
}
int main()
{
    int n;
    while(scanf("%d", &n)==1){
        for(int i=0;i<n;i++)
             scanf("%d",&a[i]);
            for(int i = 0;i < n;++i) sum[i][i] = a[i];
            for(int i=0;i<n;i++){
                for(int j=i+1;j<n;j++){
                    sum[i][j]=sum[i][j-1] + a[j];
                    if(sum[i][j]>=100)
                        sum[i][j]-=100;
                }

            }
            memset(dp,-1,sizeof dp);
             printf("%d\n",solve(0,n-1));


    }

    return 0;
}

SECOND CODE

#include <bits/stdc++.h>

using namespace std;

int a[100],sum[100][100],dp[100][100];
int solve(int ai,int bi){
    if(ai==bi){
       return 0;
    }
    if((ai+1)==bi)
        return a[ai] * a[bi];
12.         **int r= dp[ai][bi],c;**
        if(r==-1){
            for(int i=ai;i+1<=bi;i++){
                c= sum[ai][i] * sum[i+1][bi] + solve(ai,i) + solve(i+1,bi);
            if(r==-1 || c<r)r=c;
            }
        }
        return r;
}
int main()
{
    int n;
    while(scanf("%d", &n)==1){
        for(int i=0;i<n;i++)
             scanf("%d",&a[i]);
            for(int i = 0;i < n;++i) sum[i][i] = a[i];
            for(int i=0;i<n;i++){
                for(int j=i+1;j<n;j++){
                    sum[i][j]=sum[i][j-1] + a[j];
                    if(sum[i][j]>=100)
                        sum[i][j]-=100;
                }

            }
            memset(dp,-1,sizeof dp);
             printf("%d\n",solve(0,n-1));


    }

    return 0;
}