foo.cc: In function 'int main()':
foo.cc:69:16: warning: iteration 5000 invokes undefined behavior [-Waggressive-loop-optimizations]
   69 |         head[i]=-1;
      |         ~~~~~~~^~~
foo.cc:68:20: note: within this loop
   68 |     for (int i=1; i<=5001; i++) {
      |                   ~^~~~~~
foo.cc:69:16: warning: 'void* __builtin_memset(void*, int, long unsigned int)' forming offset [40008, 40015] is out of the bounds [0, 40008] of object 'head' with type 'long long int [5001]' [-Warray-bounds]
   69 |         head[i]=-1;
      |         ~~~~~~~^~~
foo.cc:20:5: note: 'head' declared here
   20 | int head[5001];
      |     ^~~~

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