Problem p00909 (C++) — buggy submission, verdict: Runtime Error

#include <cassert>
#include <iostream>
#include <stdio.h>
#include <vector>
using namespace std;

class DisjointSet {
public:
  vector<int> rank, p, w;

  DisjointSet() {}
  DisjointSet(int size) {
    rank.resize(size, 0);
    p.resize(size, 0);
    w.resize(size, 0);
    for (int i = 0; i < size; i++)
      makeSet(i);
  }

  void makeSet(int x) {
    p[x] = x;
    rank[x] = 0;
    w[x] = 0;
  }

  void unite(int x, int y) { link(findSet(x).first, findSet(y).first); }

  void uniteW(int x, int y, int v) {
    pair<int, int> px = findSet(x);
    pair<int, int> py = findSet(y);
    if (px.first == py.first)
      return;
    x = px.first;
    y = py.first;
    if (rank[x] < rank[y]) {
      p[x] = y;
      w[x] = (v - px.second + py.second);
    } else {
      p[y] = x;
      w[y] = -(v - px.second + py.second);
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }

  bool same(int x, int y) { return findSet(x).first == findSet(y).first; }
  int getDiff(int x, int y) { return findSet(x).second - findSet(y).second; }

  void link(int x, int y) {
    if (rank[x] > rank[y]) {
      p[y] = x;
    } else {
      p[x] = y;
      if (rank[x] == rank[y]) {
        rank[y] = rank[y] + 1;
      }
    }
  }

  pair<int, int> findSet(int x) {
    if (x != p[x]) {
      pair<int, int> tmp = findSet(p[x]);
      p[x] = tmp.first;
      w[x] += tmp.second;
    }
    return make_pair(p[x], w[x]);
  }
};

int main() {
  int N, M;
  char com;
  int a, b, w;

  while (1) {
    cin >> N >> M;
    if (N == 0 && M == 0)
      break;

    DisjointSet ds = DisjointSet(N);

    for (int i = 0; i < M; i++) {
      cin >> com;
      if (com == '!') {
        cin >> a >> b >> w;
        a--;
        b--;
        ds.uniteW(a, b, w);
      } else if (com == '?') {
        cin >> a >> b;
        a--;
        b--;
        if (ds.same(a, b)) {
          cout << ds.getDiff(a, b) << endl;
        } else {
          cout << "UNKNOWN" << endl;
        }
      }
    }
  }

  return 0;
}


Error: 0

Fixed submission:

#include <cassert>
#include <iostream>
#include <stdio.h>
#include <vector>
using namespace std;

class DisjointSet {
public:
  vector<int> rank, p, w;

  DisjointSet() {}
  DisjointSet(int size) {
    rank.resize(size, 0);
    p.resize(size, 0);
    w.resize(size, 0);
    for (int i = 0; i < size; i++)
      makeSet(i);
  }

  void makeSet(int x) {
    p[x] = x;
    rank[x] = 0;
    w[x] = 0;
  }

  void unite(int x, int y) { link(findSet(x).first, findSet(y).first); }

  void uniteW(int x, int y, int v) {
    pair<int, int> px = findSet(x);
    pair<int, int> py = findSet(y);
    if (px.first == py.first)
      return;
    x = px.first;
    y = py.first;
    if (rank[x] < rank[y]) {
      p[x] = y;
      w[x] = (v - px.second + py.second);
    } else {
      p[y] = x;
      w[y] = -(v - px.second + py.second);
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }

  bool same(int x, int y) { return findSet(x).first == findSet(y).first; }
  int getDiff(int x, int y) { return findSet(x).second - findSet(y).second; }

  void link(int x, int y) {
    if (rank[x] > rank[y]) {
      p[y] = x;
    } else {
      p[x] = y;
      if (rank[x] == rank[y]) {
        rank[y] = rank[y] + 1;
      }
    }
  }

  pair<int, int> findSet(int x) {
    if (x != p[x]) {
      pair<int, int> tmp = findSet(p[x]);
      p[x] = tmp.first;
      w[x] += tmp.second;
    }
    return make_pair(p[x], w[x]);
  }
};

int main() {
  int N, M;
  char com;
  int a, b, w;

  while (1) {
    cin >> N >> M;
    if (N == 0 && M == 0)
      break;

    DisjointSet ds = DisjointSet(N);

    for (int i = 0; i < M; i++) {
      cin >> com;
      if (com == '!') {
        cin >> a >> b >> w;
        ds.uniteW(a, b, w);
      } else if (com == '?') {
        cin >> a >> b;
        if (ds.same(a, b)) {
          cout << ds.getDiff(a, b) << endl;
        } else {
          cout << "UNKNOWN" << endl;
        }
      }
    }
  }

  return 0;
}