校门外的树

暴力做法：

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

using namespace std;

const int N = 10010;

int n, m;
bool st[N];

int main()
{
    scanf("%d%d", &m, &n);
    while (n -- )
    {
        int l, r;
        scanf("%d%d", &l, &r);
        for (int i = l; i <= r; i ++ ) st[i] = true;
    }

    int res = 0;
    for (int i = 0; i <= m; i ++ )
        if (!st[i])
            res ++ ;

    printf("%d\n", res);

    return 0;
}

区间并和：

#include <iostream>
#include <algorithm>

using namespace std;

typedef pair<int, int> PII;

const int N = 110;

int n, m;
PII seg[N];

int main()
{
    cin >> m >> n;

    for (int i = 0; i < n; i ++ ) cin >> seg[i].first >> seg[i].second;

    sort(seg, seg + n);

    int res = m + 1;
    int st = seg[0].first, ed = seg[0].second;
    for (int i = 1; i < n; i ++ )
        if (seg[i].first <= ed) ed = max(seg[i].second, ed);
        else
        {
            res -= ed - st + 1;//当前合并区间内树木的数量。
            st = seg[i].first, ed = seg[i].second;
        }

    res -= ed - st + 1;

    cout << res << endl;

    return 0;
}