nowcoder 203J Graph Coloring I（dfs）

#include <stdio.h>
#include<vector>
typedef std::vector<int> Vi;
#define maxn 300005
//链式存边
int fst[maxn], to[maxn << 1], nxt[maxn << 1], z = 0;
void add(int u, int v) {
    z++; to[z] = v;
    nxt[z] = fst[u]; fst[u] = z;
    z++; to[z] = u;
    nxt[z] = fst[v]; fst[v] = z;
}

int n, m;
int clr[maxn];
Vi rt;
int isodd/*是奇数环的标记*/, 
    rat/*涂色矛盾发生的地方，也就是奇数环的起点/终点*/, 
    ended/*完成了环的记录*/;
//直接涂色
void dfs(int cid) {
    int nid;
    for (int ln = fst[cid]; ln; ln = nxt[ln]) {
        nid = to[ln];
        if (clr[nid] == -1) {
            clr[nid] = !clr[cid];
            dfs(nid);
            if (ended)return;
            if (isodd) {
                rt.push_back(nid);
                if (cid == rat)//已经回退到环的起点
                    ended = 1; 
                return;
            }
        }
        else if (clr[nid] == clr[cid]) {//遇到矛盾，开始回退
            rat = nid;
            rt.push_back(nid);
            isodd = 1; ended = 0;
            return;
        }
    }
}

int main() {
    scanf("%d %d", &n, &m);
    for (int i = 1; i <= n; ++i)clr[i] = -1;
    int ai, bi;
    for (int i = 0; i<m; ++i) {
        scanf("%d%d", &ai, &bi);
        add(ai, bi);
    }
    clr[1] = 1;
    dfs(1);
    if (isodd == 0) {
        printf("0\n");
        for (int i = 1; i <= n; ++i)printf("%d ", clr[i]);
    }
    else {
        int sz = rt.size();
        //环可能会重复记录起点，去掉就好了
        if (rt[sz - 1] == rt[0])sz--;
        printf("%d\n", sz);
        for (int i = 0; i<sz; ++i)printf("%d ", rt[i]);
    }
}