duplicate zero

class Solution {
public:
    void duplicateZeros(vector<int>& arr) {
        int zcount=0;
        int i,j,temp;
        for(i=0;i<arr.size();i++){
            int a = arr[i];
            if(a==0){
                zcount++;
            }
        }

        for(i=arr.size()-1;zcount>0 & i>=0;i--){
            if(arr[i]==0)zcount=zcount-2;
            else zcount--;

          //  if(zcount<0)i++;// why this not working?

        }


        for(j=arr.size()-1;i>=0;j--,i--){
            if(arr[i]==0){
                arr[j]=0;
                j--;
            }
            arr[j]=arr[i];
        }

    }
};