倍增跑路

 数据范围，n <= 50

 具体看代码吧，倍增其实就是动态规划（目前我见过的来说）

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<vector>
using namespace std;
typedef long long ll;
int flag[70][70][70];
int map[100][100];
int main() {
	int n, m;
	scanf("%d%d", &n, &m);
	memset(map, 0x3f3f3f3f, sizeof(map));

	int be, en;
	for (int i = 0; i < m; i++) {
		scanf("%d%d", &be, &en);
		flag[be][en][0] = 1;
		map[be][en] = 1;
	}

	for (int k = 1; k <= 64; k++) {
		for (int i = 1; i <= n; i++) {
			for (int j = 1; j <= n; j++) {
				for (int t = 1; t <= n; t++) {
					if (flag[i][j][k - 1] && flag[j][t][k - 1]) {

						flag[i][t][k] = 1;
						map[i][t] = 1;
					}
				}
			}
		}
	}
	for (int k = 1; k <= n; k++) {
		for (int i = 1; i <= n; i++) {
			for (int j = 1; j <= n; j++) {
				map[i][j] = min(map[i][k] + map[k][j], map[i][j]);
			}
		}
	}
	printf("%d
", map[1][n]);
	return 0;
}