View Compilation Error

0_0_21222168_25120.cpp: In function 'void Merge(int, int)':
0_0_21222168_25120.cpp:31:13: error: 'fx' was not declared in this scope
         pre[fx]=fy;
             ^
0_0_21222168_25120.cpp:31:17: error: 'fy' was not declared in this scope
         pre[fx]=fy;
                 ^