Function to find the length of the longest balanced substring
int longestSub ( string s ) { int max_len = 0 ; int n = s . length ( ) ; for ( int i = 0 ; i < n ; i ++ ) { for ( int j = i ; j < n ; j ++ ) { if ( isValid ( s . substr ( i , j - i + 1 ) ) && max_len < j - i + 1 ) max_len = j - i + 1 ; } } return max_len ; }