What happens to the string in the operation s[left] += 1? Is it an index modification or concatenation?

In the Go language, strings are immutable. The operation s[left] += 1 is not directly applicable to strings because you cannot modify a character at a specific index.

If s is a string, attempting s[left] += 1 will result in a compile-time error.

However, if s is a byte slice ([]byte), then the operation s[left] += 1 will change the byte value at index left, increasing it by 1. This is not concatenation, but modification of a specific element in the slice.

Example:

s := []byte("abc")
s[1] += 1 // 'b' (98) becomes 'c' (99)
fmt.Println(string(s)) // outputs "acc"

In summary, the operation is impossible for strings, but for byte slices, it allows modification by index.