// Given an array of positive integers, write a function which returns
// all the unique pairs which add (equal) up to 100.
const sampleData = [0, 1, 100, 99, 0, 10, 90, 30, 55, 33, 55, 75, 50, 51, 49, 50, 51, 49, 51];
// sampleOutput = [[1,99], [0,100], [10,90], [51,49], [50,50]];

function pairs ( data ) {
  let ret = [];
  let check = [];
  data.forEach( i => {
    data.forEach( y => {
      if ( i + y === 100 ) {
        const one = `${i} ${y}`;
        const two = `${y} ${i}`;

        if ( !check.includes( one ) && !check.includes( two ) ) {
          check.push( one );

          ret.push( [ i, y ] );
        }
      }
    } );
  } );

  return ret;
}