Light bulbs（离散化+差分）

#include <bits/stdc++.h>
using namespace std;

const int MAXN = 2010;
pair<int, int> p[MAXN];
int main() {
  int T;
  int iCase = 0;
  int N, M;
  scanf("%d", &T);
  while (T--) {
    iCase++;
    scanf("%d%d", &N, &M);
    int tot = 0;
    while(M--) {
      int l, r;
      scanf("%d%d", &l, &r);
      p[tot++] = make_pair(l, 1);
      p[tot++] = make_pair(r+1, -1);
    }
    sort(p, p+tot);
    int now = 0;
    int ans = 0;
    int sum = 0;
    for (int i = 0; i < tot; i++) {
      if (now != p[i].first) {
        if (sum&1) ans += p[i].first - now;
        now = p[i].first;
      }
      sum += p[i].second;
    }
    if (sum &1) ans += N - now;
    printf("Case #%d: %d\n", iCase, ans);
  }
  return 0;
}