A Utility Function to find the common prefix between strings - str1 and str2
static string commonPrefixUtil ( string str1 , string str2 ) { string result = " " ; int n1 = str1 . Length , n2 = str2 . Length ; for ( int i = 0 , j = 0 ; i <= n1 - 1 && j <= n2 - 1 ; i ++ , j ++ ) { if ( str1 [ i ] != str2 [ j ] ) break ; result += str1 [ i ] ; } return ( result ) ; }