hey if one else to know print the anser plese say me..in the test #4 i have a empty string, but the judge say me "wa" in test #4.... why??? help me please.

#include <bits/stdc++.h>
using namespace std;

int main(){
    string cad,res;
             cin>>cad;
             res = "";
    int tam = cad.length();
    int i,j;
        i=0; j=1;

    int boolean[tam]={0};

    while(j<tam){
        if(cad[i]==cad[j]){
            boolean[i] = 1;
            boolean[j] = 1;

            if(i-1>=0){
                i-=1;
                j+=1;
            }else{
                i=j+1;
                j+=2;
            }
        }else{
            i=j;
            j+=1;
        }
    }

    for(int i=0;i<tam;i++){
         if(boolean[i]==0)
             res+=cad[i];
     }
     cout<<res;
    return 0;
}