LA 6187

只有一个地方需要注意：

设节点a的根为u，b的跟为v，则：a = u + d[a];  b = v + d[b];

已知：b-a=w。所以v - u = d[a] - d[b] + w;

在合并两个集合修改根节点时，把v的根改为u，同时v到根的距离为d[a] - d[b] + w;

#include <cstdio>
#include <cstring>

const int MAXN = 100010;

int pa[MAXN];
long long int d[MAXN];

int findset( int x )
{
    if ( pa[x] == x ) return x;
    int root = findset( pa[x] );
    d[x] += d[ pa[x] ];
    return pa[x] = root;
}

int main()
{
    int N, M;
    while ( scanf( "%d%d", &N, &M ), N || M )
    {
        for ( int i = 0; i <= N; ++i )
        {
            pa[i] = i;
            d[i] = 0;
        }
        while ( M-- )
        {
            char op[4];
            scanf( "%s", op );
            int a, b, w;
            if ( op[0] == '!' )
            {
                scanf( "%d%d%d", &a, &b, &w );
                int u = findset(a);
                int v = findset(b);
                pa[v] = u;
                d[v] = d[a] - d[b] + w;
            }
            else
            {
                scanf( "%d%d", &a, &b );
                int u = findset(a);
                int v = findset(b);
                if ( u != v ) puts("UNKNOWN");
                else printf( "%lld
", d[b] - d[a] );
            }
        }
    }
    return 0;
}