ADDED   SRM/646-U/1A.cpp
Index: SRM/646-U/1A.cpp
==================================================================
--- SRM/646-U/1A.cpp
+++ SRM/646-U/1A.cpp
@@ -0,0 +1,106 @@
+#include <iostream>
+#include <sstream>
+#include <iomanip>
+#include <vector>
+#include <string>
+#include <map>
+#include <set>
+#include <algorithm>
+#include <numeric>
+#include <iterator>
+#include <functional>
+#include <complex>
+#include <queue>
+#include <stack>
+#include <cmath>
+#include <cassert>
+#include <tuple>
+using namespace std;
+typedef long long LL;
+typedef complex<double> CMP;
+
+static const char* BAD = "XXX";
+
+class AB { public:
+	map<tuple<int,int,int>, string> memo;
+	string create(int N, int KPair, int NumB)
+	{
+		if(N==0)
+			return (KPair==0 && NumB==0 ? "" : BAD);
+
+		tuple<int,int,int> key(N, KPair, NumB);
+		if(memo.count(key))
+			return memo[key];
+
+		auto a = create(N-1, KPair-NumB, NumB);
+		if(a != BAD)
+			return memo[key] = "A" + a;
+		if(NumB>=1) {
+			auto b = create(N-1, KPair, NumB-1);
+			if(b != BAD)
+				return memo[key] = "B" + b;
+		}
+		return memo[key] = BAD;
+	}
+
+	string createString(int N, int K)
+	{
+		for(int NumB=0; NumB<=N; ++NumB) {
+			auto s = create(N, K, NumB);
+			if(s != BAD)
+				return s;
+		}
+		return "";
+	}
+};
+
+// BEGIN CUT HERE
+#include <ctime>
+double start_time; string timer()
+ { ostringstream os; os << " (" << int((clock()-start_time)/CLOCKS_PER_SEC*1000) << " msec)"; return os.str(); }
+template<typename T> ostream& operator<<(ostream& os, const vector<T>& v)
+ { os << "{ ";
+   for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
+   os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", "); os << " }"; return os; }
+void verify_case(const string& Expected, const string& Received) {
+ bool ok = (Expected == Received);
+ if(ok) cerr << "PASSED" << timer() << endl;  else { cerr << "FAILED" << timer() << endl;
+ cerr << "\to: \"" << Expected << '\"' << endl << "\tx: \"" << Received << '\"' << endl; } }
+#define CASE(N) {cerr << "Test Case #" << N << "..." << flush; start_time=clock();
+#define END	 verify_case(_, AB().createString(N, K));}
+int main(){
+
+CASE(0)
+	int N = 3; 
+	int K = 2; 
+	string _ = "ABB"; 
+END
+CASE(1)
+	int N = 2; 
+	int K = 0; 
+	string _ = "BA"; 
+END
+CASE(2)
+	int N = 5; 
+	int K = 8; 
+	string _ = ""; 
+END
+CASE(3)
+	int N = 10; 
+	int K = 12; 
+	string _ = "BAABBABAAB"; 
+END
+/*
+CASE(4)
+	int N = ; 
+	int K = ; 
+	string _ = ; 
+END
+CASE(5)
+	int N = ; 
+	int K = ; 
+	string _ = ; 
+END
+*/
+}
+// END CUT HERE

ADDED   SRM/646-U/1B-U.cpp
Index: SRM/646-U/1B-U.cpp
==================================================================
--- SRM/646-U/1B-U.cpp
+++ SRM/646-U/1B-U.cpp
@@ -0,0 +1,186 @@
+#include <iostream>
+#include <sstream>
+#include <iomanip>
+#include <vector>
+#include <string>
+#include <map>
+#include <set>
+#include <algorithm>
+#include <numeric>
+#include <iterator>
+#include <functional>
+#include <complex>
+#include <queue>
+#include <stack>
+#include <cmath>
+#include <cassert>
+#include <tuple>
+using namespace std;
+typedef long long LL;
+typedef complex<double> CMP;
+
+static const unsigned MODVAL = 1000000007;
+struct mint
+{
+	unsigned val;
+	mint():val(0){}
+	mint(int      x):val(x%MODVAL) {}
+	mint(unsigned x):val(x%MODVAL) {}
+	mint(LL       x):val(x%MODVAL) {}
+};
+mint& operator+=(mint& x, mint y) { return x = x.val+y.val; }
+mint& operator-=(mint& x, mint y) { return x = x.val-y.val+MODVAL; }
+mint& operator*=(mint& x, mint y) { return x = LL(x.val)*y.val; }
+mint operator+(mint x, mint y) { return x+=y; }
+mint operator-(mint x, mint y) { return x-=y; }
+mint operator*(mint x, mint y) { return x*=y; }
+mint POW(mint x, LL e) { mint v=1; for(;e;x*=x,e>>=1) if(e&1) v*=x; return v; }
+
+// cnt, ith
+typedef tuple<LL,LL> item;
+struct cmp {
+	bool operator()(const item& lhs, const item& rhs) {
+		LL e1,b1,e2,b2;
+		tie(e1,b1) = lhs;
+		tie(e2,b2) = rhs;
+		// b1 2^e1 > b2 2^e2 ?
+
+		LL e = abs(e1-e2);
+		if(e1==e2)
+			return b1 > b2;
+		if(e1 > e2) {
+			// b1 2^e > b2 ?
+			LL bb=b1;
+			for(int i=0; i<e; ++i) {
+				bb *= 2;
+				if(bb>b2) return true;
+			}
+			return false;
+		} else {
+			// b1 < b2 2^e ?
+			LL bb=b2;
+			for(int i=0; i<e; ++i) {
+				bb *= 2;
+				if(b1<bb) return true;
+			}
+			return false;
+		}
+	}
+};
+
+class KitayutaMart { public:
+	int lastPrice(int N, int K)
+	{
+		// Invariant: Can buy N apples with max in (K*2^(L-1), K*2^(R-1)]
+		int L=0, R=N;
+		while(R-L>1) {
+			int C=(L+R)/2;
+			(N<=max_buy(K, C) ? R : L) = C;
+		}
+		// L+1==R
+
+		// Invariant: Can buy N apples with max in (LL*2^(R-1), RR*2^(R-1)]
+		int Ll=K/2, RR=K;
+		while(RR-Ll>1) {
+			int CC=(Ll+RR)/2;
+			(N<=max_buy(K, R, CC) ? RR : Ll) = CC;
+		}
+		//Ll+1==RR
+
+		// Should be close enough. Do naively.
+		LL total = K*L + Ll;
+
+		priority_queue<item, vector<item>, cmp> Q;
+		Q.emplace(R, RR);
+		int c=R+1;
+		for(LL div=2;; div*=2,++c) {
+			total += Ll/div;
+			Q.emplace(c, Ll/div+1);
+			if(Ll/div==0)
+				break;
+		}
+
+		int ans = 0;
+		for(; total<=N; ++total) {
+			LL cnt,ith;
+			tie(cnt,ith) = Q.top();
+			Q.pop();
+
+			ans = (mint(ith)*POW(2,cnt-1)).val;
+
+			if(ith==1)
+				Q.emplace(cnt+1,ith);
+			if(ith<K)
+				Q.emplace(cnt, ith+1);
+		}
+		return ans;
+	}
+
+	LL max_buy(LL K, LL C) {
+		return max_buy(K,C,K);
+	}
+
+	LL max_buy(LL K, LL C, LL Km) {
+		// Buy many apples with max<=Km*2^(C-1) ?
+		LL total = K*(C-1) + Km;
+		for(LL div=2; Km/div>=1; div*=2) 
+			total += Km/div;
+		return total;
+	}
+};
+
+// BEGIN CUT HERE
+#include <ctime>
+double start_time; string timer()
+ { ostringstream os; os << " (" << int((clock()-start_time)/CLOCKS_PER_SEC*1000) << " msec)"; return os.str(); }
+template<typename T> ostream& operator<<(ostream& os, const vector<T>& v)
+ { os << "{ ";
+   for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
+   os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", "); os << " }"; return os; }
+void verify_case(const int& Expected, const int& Received) {
+ bool ok = (Expected == Received);
+ if(ok) cerr << "PASSED" << timer() << endl;  else { cerr << "FAILED" << timer() << endl;
+ cerr << "\to: \"" << Expected << '\"' << endl << "\tx: \"" << Received << '\"' << endl; } }
+#define CASE(N) {cerr << "Test Case #" << N << "..." << flush; start_time=clock();
+#define END	 verify_case(_, KitayutaMart().lastPrice(N, K));}
+int main(){
+
+CASE(0)
+	int N = 3; 
+	int K = 1; 
+	int _ = 4; 
+END
+CASE(1)
+	int N = 3; 
+	int K = 2; 
+	int _ = 2; 
+END
+CASE(2)
+	int N = 5; 
+	int K = 3; 
+	int _ = 4; 
+END
+CASE(3)
+	int N = 1000000000; 
+	int K = 1; 
+	int _ = 570312504; 
+END
+CASE(4)
+	int N = 987654321; 
+	int K = 876543210; 
+	int _ = 493827168; 
+END
+/*
+CASE(5)
+	int N = ; 
+	int K = ; 
+	int _ = ; 
+END
+CASE(6)
+	int N = ; 
+	int K = ; 
+	int _ = ; 
+END
+*/
+}
+// END CUT HERE