Runtime Error in CodeForces - algorithm

Question

I am trying to solve this problem : http://codeforces.com/contest/664/problem/B
Here is my code : http://ideone.com/fWgQEn
I am getting Runtime Error on Test Case 5, even though my answer is correct and I am printing it properly.
Can anyone tell me what could be the reason of this?
#include<bits/stdc++.h>
using namespace std;
int main(){
int i = 0, pos = 1, neg = 0, n;
string str;
char x;
while(1){
cin >> x;
if(x == '=') break;
else if (x == '?') continue;
else if (x == '+') pos++;
else if (x == '-') neg++;
str[i++] = x;
}
str[i] = '\0';
// cout << str[0] << str[1] << str.size() << endl;
cin >> n;
if (!(pos * n - neg >= n && pos - neg * n <= n))
cout << "Impossible" << endl;
else{
cout << "Possible\n";
int neg_sum, pos_sum;
for (int i = neg; i <= neg * n; i++){
pos_sum = n + i;
if(pos_sum <= pos * n && pos_sum >= pos) {
neg_sum = i; pos_sum = n + i;
break;
}
}
// cout << str.size() << endl;
// cout << pos_sum << " " << neg_sum << endl;
int pos_count = 1, neg_count = 0;
for(int i = -1 ; i < pos + neg - 1; i++){
// cout << "i " << i << " " << str[i] <<endl;
if(!(i + 1)){
if(pos == 1) cout << pos_sum << " ";
else cout << pos_sum / (pos - 1) << " ";
}
else{
if(str[i] == '+'){
if(pos_count++ != pos -1) cout << "+ "<< pos_sum / (pos - 1) << " ";
else cout << "+ "<< pos_sum % (pos - 1) << " ";
}
else{
if(neg == 1) cout << "- " << neg_sum << " ";
else if(neg_count++ != neg -1) cout << "- "<< neg_sum / (neg - 1) << " ";
else cout << "+ "<< neg_sum % (neg - 1) << " ";
}
}
}
cout << "= " << n;
}
return 0;
}
TIA!!

Answer 1

string str;
char x;
while(1){
cin >> x;
if(x == '=') break;
else if (x == '?') continue;
else if (x == '+') pos++;
else if (x == '-') neg++;
str[i++] = x;
}
I think at least str[i++] = x; will run into un-allocated spaces and cause undefined behavior. See
http://www.cplusplus.com/reference/string/string/operator[]/
Try
str += x; i++;
instead.