[BZOJ4404] [Neerc2015]Binary vs Decimal（BFS）

#include <bits/stdc++.h>
using namespace std;

const int maxl = 180;
const int modn = 1e9 + 7;
const int maxn = 10500;
bitset<maxl> xp[maxl];

void add(bitset<maxl> &a, bitset<maxl> b) {
    int carry = 0;
    for (int i = 0; i < maxl; ++i) {
        int t = carry + (int)a.test(i) + (int)b.test(i);
        a.set(i, t % 2);
        carry = t / 2;
    }
}

struct Node {
    Node() { len = 0; }
    bool set() {
        //cout << len << endl;
        s.set(len - 1);
        bitset<maxl>& c = xp[len-1];
        int carry = 0;
        for (int i = 0; i < maxl; ++i) {
            int t = carry + (int)b.test(i) + (int)c.test(i);
            if (i < len && t % 2 != s.test(i))
                return false;
            b.set(i, t % 2);
            carry = t / 2;
        }
        return true;
    }
    bitset<maxl> s;
    bitset<maxl> b;
    int len;
};

queue<Node> q;
vector<string> ans[maxl];
int anscnt;

void insert_ans(Node &x) {
    if (!x.s.test(x.len - 1)) return;
    char res[maxl];
    memset(res, 0, sizeof(res));
    for (int i = 0; i < x.len; ++i)
        res[x.len-i-1] = x.s.test(i) + ‘0‘;
    anscnt++;
    ans[x.len].push_back(res);
}

string find_string(int rank) {
    int tot = 0;
    for (int i = 0; i < maxl; ++i) {
        if (tot + ans[i].size() >= rank) {
            sort(ans[i].begin(), ans[i].end());
            return ans[i][rank-tot-1];
        } else tot += ans[i].size();
    }
}

int main()
{
    #ifdef ULTMASTER
    freopen("a.in", "r", stdin);
    #endif
    int N; cin >> N;
    xp[0].set(0);
    for (int i = 1; i < maxl; ++i) {
        xp[i] = (xp[i-1]<<1);
        add(xp[i], xp[i]<<2);
    }
    Node zero; zero.len = 1;
    Node one = zero; one.set();
    q.push(zero); q.push(one);
    while (!q.empty()) {
        if (anscnt > N + 200)
            break;
        Node u = q.front();
        insert_ans(u);
        q.pop(); u.len++;
        q.push(u);
        if (u.set()) q.push(u);
    }
    cout << find_string(N) << endl;
    return 0;
}