param { number [ ] } numbers return { number } function FindNumberAppearOnce ( numbers ) { if ( ! numbers || numbers . length < 2 ) { return - 1 ; } let resultOR = 0 ; let len = numbers . length ; for ( let i = 0 ; i < len ; i ++ ) { resultOR = resultOR ^ numbers [ i ] ; } return resultOR ; } export { FindNumberAppearOnce } ;