foo.cc: In function 'void bfs()':
foo.cc:29:30: warning: iteration 19 invokes undefined behavior [-Waggressive-loop-optimizations]
                     fa[j][k] = fa[fa[j][k - 1]][k - 1];
                     ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:28:34: note: within this loop
                 for(int k = 1; k <= 20; k++)
                                ~~^~~~~

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