Find four missing numbers in an array containing elements from 1 to N | Java program to find missing 4 elements in an array of size N where elements are in range from 1 to N + 4. ; Finds missing 4 numbers in O ( N ) time and O ( 1 ) auxiliary space . ; To keep track of 4 possible numbers greater than length of input array In Java , helper is automatically initialized as 0. ; Traverse the input array and mark visited elements either by marking them as negative in arr [ ] or in helper [ ] . ; If element is smaller than or equal to length , mark its presence in arr [ ] ; Mark presence in helper [ ] ; Print all those elements whose presence is not marked . ; Driver code
class Missing4 { public static void missing4 ( int [ ] arr ) { int [ ] helper = new int [ 4 ] ; for ( int i = 0 ; i < arr . length ; i ++ ) { int temp = Math . abs ( arr [ i ] ) ; if ( temp <= arr . length ) arr [ temp - 1 ] *= ( - 1 ) ; else if ( temp > arr . length ) { if ( temp % arr . length != 0 ) helper [ temp % arr . length - 1 ] = - 1 ; else helper [ ( temp % arr . length ) + arr . length - 1 ] = - 1 ; } } for ( int i = 0 ; i < arr . length ; i ++ ) if ( arr [ i ] > 0 ) System . out . print ( i + 1 + " ▁ " ) ; for ( int i = 0 ; i < helper . length ; i ++ ) if ( helper [ i ] >= 0 ) System . out . print ( arr . length + i + 1 + " ▁ " ) ; return ; } public static void main ( String [ ] args ) { int [ ] arr = { 1 , 7 , 3 , 12 , 5 , 10 , 8 , 4 , 9 } ; missing4 ( arr ) ; } }