蒟蒻问一个小问题，我为什么赋值不过去（太奇怪了QAQ）

#include<cstdio>
#include<iostream>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
#include<vector>
#include<queue>
#include<map>
#include<stack>
#include<set>
using namespace std;
typedef long long ll;

#define maxn 200005

int ltp, LTP, lk[maxn], LK[maxn], n, m, w, u, v, s, t, cnt[maxn], a[maxn];
bool f[maxn], vis[maxn], b, bl[maxn], ans[maxn];

struct edg {
	int y, nxt;
} e[maxn];

edg E[maxn];

void ist(int x, int y) {
	e[++ltp] = {y, lk[x]};
	lk[x] = ltp;
}

void IST(int X, int Y) {
	E[++LTP] = {Y, LK[X]};
	LK[X] = LTP;
}

void DFS(int X) {
	if(vis[X]) return;
	vis[X] = 1;
	for(int i = LK[X]; i; i = E[i].nxt)
		DFS(E[i].y);
}

void bfs_(int x) {
	queue<int> q;
	q.push(x);
	f[x] = 1;
	while(!q.empty()) {
		int z = q.front();
		q.pop();
		for(int i = lk[z]; i; i = e[i].nxt) {
			if(!f[e[i].y]) {
				int p = e[i].y;
				for(int j = lk[p]; j; j = e[j].nxt) {
					if(!vis[e[j].y]) {
						b = 1;
						f[e[i].y] = 1;
						bl[e[i].y] = 0;
						break;
					}
				}
				if(!b) {
					q.push(e[i].y);
					f[e[i].y] = 1;
				}
			}
		}
	}
}

void bfs__(int x) {
	queue<int> q;
	q.push(x);
	bl[x] = 1;
	while(!q.empty()) {
		int z = q.front();
		if(z == t) return;
		q.pop();
		for(int i = lk[z]; i; i = e[i].nxt) {
			int p = e[i].y;
			if(bl[p]) {
				q.push(p);
				printf("p = %d, ans[%d] = %d, z = %d, ans[%d] = %d\n", p, p, ans[p], z, z, ans[z]);
				ans[p] = ans[z]+1;
				printf("p = %d, ans[%d] = %d, z = %d, ans[%d] = %d\n", p, p, ans[p], z, z, ans[z]);
			}
		}
	}
}

int main() {
	cin >> n >> m;
	for(int i = 1; i <= m; ++i) {
		scanf("%d %d", &u, &v);
		IST(v, u);
		ist(u, v);
	}
	cin >> s >> t;
	DFS(t);
	for(int i = 1; i < maxn; ++i)
		bl[i] = vis[i];
	for(int i = 1; i <= n; ++i) {
		for(int j = lk[i]; j; j = e[j].nxt) {
			int p = e[j].y;
			if(!vis[p]) {
				bl[i] = 0;
				break;
			}
		}
	}
	for(int i = 1; i <= n; ++i)
		printf("bl[%d] = %d\n", i, bl[i]);
	memset(f, 0, sizeof(f));
	bfs__(s);
	if(ans[t] == 0) ans[t] = -1;
	cout << ans[t];
	return 0;
}