Go language Rune Method substring error problem

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A substring method on the Internet:

Func SubString (str string,begin,length int) (substr string) {//converts a string into []runers: = []rune (str) Lth: = Len (RS)//simple cross-border judgment if Begin < 0 {begin = 0}if begin >= Lth {begin = Lth}end: = begin + Lengthif End > lth {end = lth}//returns substring return stri Ng (Rs[begin:end])}

I used to find that the interception of the string when the failure problem, and then carefully read some of his code, found that there are rune, modified some rune removed after the real can intercept the string, the code is as follows:

Func SubString (str string,begin,length int) (substr string) {lth: = Len (str)//simple cross-border judgment if begin < 0 {begin = 0}if begin >= lth {begin = Lth}end: = begin + Lengthif End > lth {end = lth}//returns substring return string (Str[begin:end])}