Jakarta Asia 2018 做题记录

D.

细节题。

#include <bits/stdc++.h>

using namespace std;

const int MAXN = 500 + 10;

char a[MAXN][MAXN];

int n, m;

int main() {
    cin >> n >> m;
    for (int i = 0; i < n; ++i)
        cin >> a[i];
    int ans = 0;
    if (n == 1) {
        for (int i = 1; i < m - 1; ++i)
            if (a[0][i] == '.')
                ++ans;
    } else if (m == 1) {
        for (int i = 1; i < n - 1; ++i)
            if (a[i][0] == '.')
                ++ans;
    } else if (n == 2) {
        for (int i = 1; i < m - 1; ++i)
            if (a[0][i] == '.' && a[1][i] == '.')
                ++ans;
    } else if (m == 2) {
        for (int i = 1; i < n - 1; ++i)
            if (a[i][0] == '.' && a[i][1] == '.')
                ++ans;
    } else {
        for (int i = 1; i < n - 1; ++i)
            for (int j = 1; j < m - 1; ++j) {
                if (a[i][j] == '.')
                    ++ans;
            }
        int add = 1;
        for (int i = 1; i < n - 1; ++i) {
            if (a[i][0] == '#' || a[i][m - 1] == '#')
                add = 0;
        }
        for (int i = 1; i < m - 1; ++i) {
            if (a[0][i] == '#' || a[m - 1][i] == '#')
                add = 0;
        }
        ans += add;
    }
    cout << ans << endl;
}

J.

签到题。贪心

#include <bits/stdc++.h>

using namespace std;

string b2s(int64_t b) {
    string s = "";
    while (b) {
        if (b & 1)
            s.push_back('1');
        else
            s.push_back('0');
        b >>= 1;
    }
    reverse(s.begin(), s.end());
    return s;
}

int64_t s2b(const string& s) {
    int64_t b = 0;
    for (auto x : s) {
        b = b * 2 + x - '0';
    }
    return b;
}

int main() {
    ios::sync_with_stdio(false);

    int64_t n;
    string bits;
    cin >> n;
    cin >> bits;
    int cnt = 0;
    while (s2b(bits) > n) {
        for (size_t i = 1; i < bits.size(); ++i) {
            if (bits[i] == '1') {
                bits.erase(bits.begin() + i);
                goto END_OF_WHILE;
            }
        }
        bits.pop_back();
      END_OF_WHILE:
        cnt++;
    }
    cout << cnt << endl;
}

K.

对图 dfs，过程贪心选择。可以证明是最优解。

#include <bits/stdc++.h>

using namespace std;

const int MAXN = 100000 + 10;

int dfn[MAXN], alloc;
struct Tuple {
    int a, b, c;
    Tuple(int a=0, int b=0, int c=0): a(a), b(b), c(c) {}
};
vector<Tuple> ans;

struct Graph {
    struct Edge {
        int v, n;
    } e[MAXN << 1];
    int G[MAXN], edgeCnt;
    void _add_edge(int u, int v) {
        e[++edgeCnt] = (Edge){v, G[u]};
        G[u] = edgeCnt;
    }
    void add_edge(int u, int v) {
        _add_edge(u, v);
        _add_edge(v, u);
    }
    bool dfs(int u, int par=-1) {
        dfn[u] = ++alloc;
        int temp = -1;
        for (int i = G[u]; i; i = e[i].n) if (e[i].v != par) {
            if (dfn[e[i].v]) {
                if (dfn[e[i].v] > dfn[u])
                    continue;
                if (~temp) {
                    ans.push_back(Tuple(temp, u, e[i].v));
                    temp = -1;
                } else {
                    temp = e[i].v;
                }
            } else {
                if (dfs(e[i].v, u)) {
                    if (~temp) {
                        ans.push_back(Tuple(temp, u, e[i].v));
                        temp = -1;
                    } else {
                        temp = e[i].v;
                    }
                }
            }
        }
        if (temp == -1) {
            return true;
        }
        if (par != -1) {
            ans.push_back(Tuple(temp, u, par));
        }
        return false;
    }
} graph;

int main() {
    ios::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    for (int i = 1; i <= m; ++i) {
        int u, v;
        cin >> u >> v;
        graph.add_edge(u, v);
    }
    for (int i = 1; i <= n; ++i) {
        if (!dfn[i]) {
            graph.dfs(i);
        }
    }
    cout << ans.size() << endl;
    for (auto x : ans) {
        cout << x.a << ' ' << x.b << ' ' << x.c << endl;
    }
}