cf19E. Fairy(奇环 二分图染色)

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 1e5 + 10;
inline int read() {
    char c = getchar(); int x = 0, f = 1;
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
} 
int N, M, Pre[MAXN], even[MAXN], odd[MAXN], col[MAXN], cly, last, ans[MAXN], dep[MAXN];
struct Edge {
    int u, v, id, nxt;
}E[MAXN];
int head[MAXN], num;
void AddEdge(int x, int y, int id) {
    E[num] = (Edge) {x, y, id, head[x]};
    head[x] = num++;
}
void dfs(int x, int fa) {
    col[x] = col[fa] ^ 1; dep[x] = dep[fa] + 1;
    for(int i = head[x]; ~i; i = E[i].nxt) {
        int to = E[i].v;
        if(col[to] == -1) {
            Pre[to] = i; dfs(to, x);
            even[x] += even[to];
            odd[x] += odd[to];
        } else if(dep[to] + 1 < dep[x]){
            if(col[to] == col[x]) last = i, cly++, odd[x]++, odd[to]--;
            else even[x]++, even[to]--;
        }
    }
}
int main() {
    memset(head, -1, sizeof(head));
    N = read(); M = read(); 
    for(int i = 1; i <= M; i++) {
        int x = read(), y = read();
        AddEdge(x, y, i); AddEdge(y, x, i);
    }
    memset(col, -1, sizeof(col)); col[0] = 0;
    for(int i = 1; i <= N; i++) if(col[i] == -1) dfs(i, 0);
    if(cly == 0) {
        printf("%d\n", M);
        for(int i = 1; i <= M; i++) printf("%d ", i);
        return 0;
    }
    if(cly == 1) ans[E[last].id] = 1;
    for(int i = 1; i <= N; i++) if(odd[i] == cly && !even[i]) ans[E[Pre[i]].id] = 1;
    int cnt = 0;
    for(int i = 1; i <= M; i++) if(ans[i]) cnt++;
    printf("%d\n", cnt);
    for(int i = 1; i <= M; i++) if(ans[i]) printf("%d\n", i);
    return 0;
}