Two sum: Difference between revisions
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Nesting concatMap yields the cartesian product of the list with itself, and |
Nesting concatMap yields the cartesian product of the list with itself, and |
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functions passed to Array.map() have access to the array index in their second argument. |
functions passed to Array.map() have access to the array index in their second argument. |
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Returning [] where the y index is lower than |
Returning [] where the y index is lower than or equal to the x index ignores the 'lower triangle' |
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of the cartesian grid, skipping mirror-image and duplicate number pairs. |
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Returning [] where a sum condition is not met similarly acts as a filter – all of the empty lists |
Returning [] where a sum condition is not met similarly acts as a filter – all of the empty lists |
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in the map are eliminated by the concat. |
in the map are eliminated by the concat. |
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return concatMap(function (x, ix) { |
return concatMap(function (x, ix) { |
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return concatMap(function (y, iy) { |
return concatMap(function (y, iy) { |
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return iy < ix ? [] : x + y === n ? [ |
return iy <= ix ? [] : x + y === n ? [ |
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[ix, iy] |
[ix, iy] |
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] : [] |
] : [] |
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