# Check if a binary string has a 0 between 1s or not in C++

Here we will see one interesting problem. We have to check whether a string has 0 in between a 1s or not. If not, then the string is valid, otherwise invalid.Suppose there are three strings −

• 10001111010
• 00001111100
• 01111101111

From these three strings, only B is valid, because there is no 0 inside the stream of 1s

To solve this problem, we will find the index of first 1 present in the string, and also find the index of the last 1. Then we will check, is there any 0 from these two indices, if so, then return false, otherwise true (as valid)

## Example

Live Demo

#include <iostream>
using namespace std;
bool hasZeroInOnes(string str) {
int first, last;
for(first = 0; first < str.length(); first++){
if(str[first] == '1')
break;
}
for(last = str.length() - 1; last >= 0; last--){
if(str[last] == '1')
break;
}
for(int i = first+1; i < last; i++){
if(str[i] == '0')
return false;
}
return true;
}
int main() {
string str = "00001111100";
if(hasZeroInOnes(str)){
cout << str << " is a valid string";
} else {
cout << str << " is NOT a valid string";
}
}

## Output

00001111100 is a valid string