Swfit1 String截取方法：

var s="abcdefghijk"

let index = advance(s.startIndex,5)
let index2 = advance(s.endIndex,-6)

let  start =  advance(s.startIndex,2)
let  end =  advance(s.startIndex,6)
s.substringToIndex(index2)

s.substringFromIndex(advance(s.startIndex,5))

//截取  第三个往后
s.substringFromIndex(advance(s.startIndex,3))

//截取  第 N -M 个
s.substringWithRange(Range(start: start,end: end))

swift 2 方法：

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