Two sum: Difference between revisions

===ES5===

Nested calls to concatMap yield the cartesian product of the list with itself, and

functions passed to Array.map() have access to the array index in their second argument.

Returning [] where the y index is lower than then x index ignores the 'lower triangle' of the cartesian grid,

so mirror-image number pairs are not considered.

Returning [] where the sum is not that required similarly acts as a filter – all of the empty lists

in the map are eliminated by the concat.

<lang JavaScript>(function () {

var concatMap = function (f, a) {

return [].concat.apply([], a.map(f))

};

return function (n, xs) {

return concatMap(function (x, ix) {

return concatMap(function (y, iy) {

return iy < ix ? [] : x + y === n ? [

[ix, iy]

] : []

}, xs)

}, xs)

}(21, [0, 2, 11, 19, 90]);

})();

</lang>

{{Out}}

<lang JavaScript>[[1,3]]</lang>