Petrozavodsk Winter Camp, Day 8, 2014, Mosaic

给你三个数字a，b，c，让你用1～m的数字凑出来

结论：有2个1和2个2肯定凑不出来，然后就搜索

#include <bits/stdc++.h>
using namespace std;
#define rep(i, j, k) for (int i = int(j); i <= int(k); ++ i) 
#define dwn(i, j, k) for (int i = int(j); i >= int(k); -- i)
typedef long long LL;
typedef pair<int, int> P;
const int N = 3e5 + 7;
char col[N];
int dfs(int i, int j, int k, int x) {
    if (i == 0 && j == 0 && k == 0) return 1;
    if (i < 0 || j < 0 || k < 0 || x <= 0) return 0;
    col[x] = 1;
    if (dfs(i - x, j, k, x - 1)) return 1;
    col[x] = 2;
    if (dfs(i, j - x, k, x - 1)) return 1;
    col[x] = 3;
    if (dfs(i, j, k - x, x - 1)) return 1;
    return 0;
}
int main() {
    ios::sync_with_stdio(0);
    LL a, b, c;
    cin >> a >> b >> c;
    LL n = a + b + c;
    LL x = 1;
    for (x = 1; n > 0; x ++) n -= x; x --; int xx = x;
    int c1 = (a == 1) + (b == 1) + (c == 1);
    int c2 = (a == 2) + (b == 2) + (c == 2);
    if (c1 >= 2 || c2 >= 2 || n != 0) {
        printf("Impossible
");
        return 0;
    }
    dfs(a, b, c, x);
    // cout << a << ' ' << b << ' ' << c << ' ' << x << '
';
    auto tran = [&](int x) -> char {
        if (x == 1) return 'W';
        if (x == 2) return 'G';
        return 'B';
    };
    rep(i, 1, xx) printf("%c", tran(col[i]));
}
/*
11 5 5
*/