Connected graph (day 72)

1： subject

 Given an undirected graph and all its edges , Determine whether all vertices of the graph are connected .

 Input format 
 The input contains several sets of data .

 The first row of each set of data contains two integers  n  and  m, Represents the number of points and edges of an undirected graph .

 Next  m  That's ok , Each line contains two integers  x,y, Indication point  x  Sum point  y  Connected to a .

 The number of the point is from  1  To  n.

 There may be double edges and self rings in the graph .

 Output format 
 Output one row per group of data , A result , If all the vertices are connected , Output  YES, Otherwise output  NO.

 Data range 
 The input can contain at most  10  Group data .
1≤n≤1000,
1≤m≤5000,
1≤x,y≤n
 sample input ：
4 3
1 2
2 3
3 2
3 2
1 2
2 3
 sample output ：
NO
YES

2： Code implementation

#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 1010;

int n, m;
int p[N];

int find(int x)
{
    
    if (p[x] != x) p[x] = find(p[x]);
    return p[x];
}

int main()
{
    
    while (cin >> n >> m)
    {
    
        for (int i = 1; i <= n; i ++ ) p[i] = i;

        while (m -- )
        {
    
            int a, b;
            cin >> a >> b;
            p[find(a)] = find(b);
        }

        bool res = true;
        for (int i = 2; i <= n; i ++ )
            if (find(i) != find(1))
            {
    
                res = false;
                break;
            }

        if (res) puts("YES");
        else puts("NO");
    }

    return 0;
}