foo.cc: In function 'void DP(int)':
foo.cc:28:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < tree[u].size(); ++i) {
                     ~~^~~~~~~~~~~~~~~~
foo.cc: In function 'void init()':
foo.cc:14:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d %d", &n, &m); //n为共有n门课程，需要选择m门课程
     ~~~~~^~~~~~~~~~~~~~~~~
foo.cc:17:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%d %d",&y,&f[i][1]);
         ~~~~~^~~~~~~~~~~~~~~~~~~~~

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