#include <stdio.h>
//a{n} = 2a{n-1} + a{n-2}, a{1}=2, a{2}=3(再帰なし版)
int main(void) {
int n = 4;
int a=2, b = 3, c;
for(int i = 3; i <= n; i++){
c = 2*b + a;
a = b;
b = c;
}
return 0;
}
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