foo.cpp:12:31: error: size of array 'b' is too large
 int a[100001]={0},b[1500000001]={0};
                               ^
foo.cpp: In function 'int main()':
foo.cpp:21:9: error: 'b' was not declared in this scope
         b[a[i]]++; 
         ^
foo.cpp:24:12: error: 'b' was not declared in this scope
         if(b[i]) printf("%d %d\n",i,b[i]); 
            ^