/in/foo.cc: In function 'int main()':
/in/foo.cc:66:35: warning: iteration 19 invokes undefined behavior [-Waggressive-loop-optimizations]
     for(int i=1;i<=20;i++) if(de[i]) m=i; //求最大颜色编号
                               ~~~~^
/in/foo.cc:66:18: note: within this loop
     for(int i=1;i<=20;i++) if(de[i]) m=i; //求最大颜色编号
                 ~^~~~