"Algorithm interesting" produces random DNA sequences (strings)

"topic description" to write a program (unlimited language), can produce random DNA sequence (string) as required.

DNA sequences consist of four bases (characters) of a, T, C and G. It is required to produce a DNA sequence of 100 bases in the proportion of a 10%, T 20%, C 30%, G 40%.

Pay attention to the random degree of good, and meet the proportional requirements.

I first: (Everyone should not look at my program, otherwise it may affect your own thinking)

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