nc16649 校门外的树 差分

差分，tr[i]为0则说明有树。

#include <iostream>
#include <cstring>
using namespace std;
int tr[10010];
int main()
{
    int l, m;
    memset(tr, 0, sizeof(tr));
    cin >> l >> m;
    for (int i = 1; i <= m; i++)
    {
        int l, r;
        cin >> l >> r;
        tr[l]--;
        tr[r+1]++;
    }
    int cnt = 0;
    if (tr[0] == 0) cnt++;
    for (int i = 1; i <= l; i++)
    {
        tr[i] += tr[i - 1];
        if (tr[i] == 0) cnt++;
    }
    cout << cnt;
    return 0;
}