Swift3 ) 2차원배열을 1차원으로 만드는 방법(Flatten a Array of Arrays in Swift)

2차원배열을 1차원으로 만드는 방법 (Flatten a Array of Arrays in Swift)

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let arr : [[Int]] = [[1,2,3],[4,5,6],[7,8,9]]
let useJoined = Array(arr.joined())
let useflatMap = arr.flatMap{$0}
let useReduce = arr.reduce([],+)
//[1,2,3,4,5,6,7,8,9]