为什么 $1 \sim N$ 枚举 TLE 90pts，$N \sim 1$ 枚举 AC

TLE 90pts：

#include <bits/stdc++.h>

#define IOS ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define ll long long
#define ul unsigned long long

using namespace std;

vector<int> v[114514];

int A[1919810];

void dfs(int p, int w) {
	if (A[p] >= w) return;
	A[p] = w;
	for (auto i : v[p])
		dfs(i, w);
}

int main() {
	int N, M;
	scanf ("%d%d", &N, &M);
	for (int i = 1; i <= M; ++i) {
		int x, y;
		scanf ("%d%d", &x, &y);
		v[y].push_back(x);
	}
	for (int i = 1; i <= N; ++i) dfs(i, i);
	for (int i = 1; i <= N; ++i) printf ("%d ", A[i]);
}

AC：

#include <bits/stdc++.h>

#define IOS ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define ll long long
#define ul unsigned long long

using namespace std;

vector<int> v[114514];

int A[1919810];

void dfs(int p, int w) {
	if (A[p]) return;
	A[p] = w;
	for (auto i : v[p])
		dfs(i, w);
}

int main() {
	int N, M;
	scanf ("%d%d", &N, &M);
	for (int i = 1; i <= M; ++i) {
		int x, y;
		scanf ("%d%d", &x, &y);
		v[y].push_back(x);
	}
	for (int i = N; i >= 1; --i) dfs(i, i);
	for (int i = 1; i <= N; ++i) printf ("%d ", A[i]);
}