31/10+311/100+3111/1000+31111/10000

31/10+311/100+3111/1000+31111/10000
=3+1/10+3+11/100+3+111/1000+3+1111/10000
=12+(1/10+11/100+111/1000+1111/10000)
=12+4321/10000
=12又4321/10000
如果要化成小数，则=12.4321

这好像不需要什么算法。
可以换成小数 =12.4321.

31/10+311/100+3111/1000+31111/10000
＝31/10+31.1/10+31.11/10+31.111/10
＝31/10+（31+0.1）/10+（31+0.1+0.01）/10+（31+0.1+0.01+0.001）/10
＝31/10+[31/10+0.1/10]+[31/10+0.1/10+0.01/10]+[31/10+0.1/10+0.01/10+0.001/10]
＝4×（31/10）+3×（0.1/10）+2×（0.01/10）+0.001/10
＝12.4+0.03+0.002+0.0001
＝12.4321

12.4321.

31/10+311/100+3111/1000+31111/10000
=3.1+3.11+3.111+3.1111
=3*4+(0.1+0.11+0.111+0.1111)
=12+0.4321
=12.4321

不难啊，12又4321/10000 或12.4321