不确定思路错还是代码错 思路：如果数据有解就先构造生成树，然后随便找一个不在树上的边，如果边上两点是祖先后代关系就将祖先以下（不含）染成黑色，其余全部染成白色，如果不是祖先后代关系就将二者的子树（含）都染成黑色，其余全部白色 代码：

#include <iostream>
#include <vector>

// #ifndef ONLINE_JUDGE
// #define ONLINE_JUDGE
// #endif

#ifdef ONLINE_JUDGE
#define TEST
#endif
#ifndef TEST
#define DEBUG
#endif

typedef long long ll;

using namespace std;

#ifdef DEBUG
constexpr bool debug = true;
#else
constexpr bool debug = false;
#endif

constexpr int maxn = 200010;

class UnionSets {
private:
    int fa[maxn];
public:
    void init(const int n) {
        for (int i = 1; i <= n; ++i)
            fa[i] = i;
    }
    int find(const int x) {
        return fa[x] == x ? x : fa[x] = find(fa[x]);
    }
    void merge(const int x, const int y) {
        fa[find(x)] = find(y);
    }
}sts;

int colors[maxn];
vector<int> V[maxn];

void dfs(const int x, const int fa, bool color, const int exu, const int exv) {
    if (x == exu || x == exv) {
        if (color)
            colors[exu ^ exv ^ x] = false;
        color = true;
    }
    colors[x] = color;
    for (int v : V[x]) {
        if (v == fa)
            continue;
        dfs(v, x, color, exu, exv);
    }
}

int main() {
    #ifdef TEST
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    #endif

    // type in your program here
    int T, n, m;
    cin >> T;
    while (T--) {
        bool extra;
        int u, v, exu, exv;
        
        cin >> n >> m;

        sts.init(n);
        for (int i = 1; i <= n; ++i)
            V[i].clear();
        
        extra = false;
        while (m--) {
            cin >> u >> v;
            if (sts.find(u) == sts.find(v))
                extra = true, exu = u, exv = v;
            else {
                sts.merge(u, v);
                V[u].push_back(v);
                V[v].push_back(u);
            }
        }
        
        if (n <= 2) {
            cout << "-1\n";
            continue;
        }
        
        bool impossible = false;
        for (int i = 2; i <= n; ++i)
            if (sts.find(i) != sts.find(1))
                impossible = true;
        if (impossible || !extra) {
            cout << "-1\n";
            continue;
        }
        
        const bool hv1 = (exu == 1 || exv == 1);
        const bool hv2 = (exu == 2 || exv == 2);
        if (!hv1) dfs(1, 0, 0, exu, exv);
        else if (!hv2) dfs(2, 0, 0, exu, exv);
        else dfs(3, 0, 0, exu, exv);
        
        for (int i = 1; i <= n; ++i)
            cout << (colors[i] ? 'B' : 'W');
        cout << '\n';
    }

    #ifndef TEST
    while (true);
    #endif

    return 0;
}