Find n | PHP program to find n - th number containing only 4 and 7. ; If n is odd , append 4 and move to parent ; If n is even , append 7 and move to parent ; Reverse res and return . ; Driver code
< ? php function findNthNo ( $ n ) { $ res = " " ; while ( $ n >= 1 ) { if ( $ n & 1 ) { $ res = $ res . "4" ; $ n = ( int ) ( ( $ n - 1 ) / 2 ) ; } else { $ res = $ res . "7" ; $ n = ( int ) ( ( $ n - 2 ) / 2 ) ; } } return strrev ( $ res ) ; } $ n = 13 ; echo findNthNo ( $ n ) ; ? >