shuffle_string.rb

# @param {String} s
# @param {Integer[]} indices
# @return {String}
def restore_string(s, indices)
    result = '' * indices.size
    indices.each_with_index do |item, index|
        result[item] = s[index]
    end
    result
end

Without creating a string of the required size, you will get an error.

A bit confusing, cleanup:

def restore_string(s, indices)
  result = '0' * (s.size)
  indices.each_with_index do |target_index, index|
    result[target_index] = s[index]
  end
  result
end

It's easy to get confused. The indices where the string needs to be copied is provided, but the index where we need to copy the character from the given string is different.