int fm(int a,int b) {if(b==1)return a; else return

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fm(4,3)
b=3≠1return a+fm(a,b-1)=4+fm(4,2)
fm(4,2)
b=2≠1return a+fm(a,b-1)=4+fm(4,1)
fm(4,1)
b=1return a
∴fm(4,1)=4
f(4,2)=4+4=8
f(4,3)=4+8=12
最后输出12