Gym 100507h-pair:normal and Paranormal

Title Link: http://codeforces.com/gym/100507/attachments

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When I first saw it, it felt like an interval $dp$. However the complexity has been stuck in $o (n^3) $ optimization does not come down

And then I tried, and some of the greedy ones hung up on the bigger data.

Repeatedly thinking of uppercase letters and corresponding lowercase letters match to think of the previously done brace matching

Only the left and right relationship between the uppercase and lowercase letters is unlimited.

So you can use a stack to help greedy

If the new letters are currently added, they can be matched directly to the top of the stack.

Otherwise, it will be left on top of the stack (to prevent the matching line from being limited to match the letters between the matching lines).

1#include <cstdio>2#include <cstring>3#include <cmath>4#include <algorithm>5 using namespacestd;6 Const intN =10010;7 CharS[n];8 intSta[n];9 intMa[n], ans[n], num[n];Ten intN, Len, top; One intMain () A { -scanf"%d%s", &n, S +1); -Len = n <<1; the      for(inti =1; I <= Len; ++i) -         if(!top | | ABS (int) S[sta[top-1]]-s[i])! = +) -sta[top++] =i; -         Else +         { -Ma[i] = sta[top-1]; +Ma[sta[top-1]] =i; A--top; at         } -     if(top) -     { -Puts"Impossible"); -         return 0; -     } in      for(inti =1; I <= Len; ++i) -         if(S[i] <'a') toNum[i] = num[i-1]; +         Else -Num[i] = num[i-1] +1; the      for(inti =1; I <= Len; ++i) *         if(S[i] <'a') $printf"%d", Num[ma[i]]);Panax Notoginseng}

Gym 100507h-pair:normal and paranormal