foo.cc: In function 'void dfs(int, int)':
foo.cc:16:22: warning: iteration 19 invokes undefined behavior [-Waggressive-loop-optimizations]
             fa[u][k] = fa[fa[u][k - 1]][k - 1];
             ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:15:25: note: within this loop
        for(int k = 1; k <= 20; k++)
                       ~~^~~~~

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