Efficiently find first repeated character in a string without using any additional data structure in one traversal | Returns - 1 if all characters of str are unique . Assumptions : ( 1 ) str contains only characters from ' a ' to ' z ' ( 2 ) integers are stored using 32 bits ; An integer to store presence / absence of 26 characters using its 32 bits . ; If bit corresponding to current character is already set ; set bit in checker ; Driver code
function FirstRepeated ( str ) { let checker = 0 ; for ( let i = 0 ; i < str . length ; ++ i ) { let val = ( str [ i ] - ' ' ) ; if ( ( checker & ( 1 << val ) ) > 0 ) return i ; checker |= ( 1 << val ) ; } return - 1 ; } let s = " " ; let i = FirstRepeated ( s ) ; if ( i != - 1 ) document . write ( " " + s [ i ] + " " + i ) ; else document . write ( " " ) ;