算法1

区间修改的线段树

(线段树) $O(nlogn)$

#include <iostream>
#define lson u << 1
#define rson u << 1 | 1
using namespace std;

const int N = 10002;

int L, m;

struct node {
    int l, r;
    int val;
    bool tag;
}t[N << 2];

void pushup(int u) {
    t[u].val = t[lson].val + t[rson].val;
}

void pushdown(int u) {
    if (t[u].tag) {
        t[u].tag = false;
        t[lson].tag = true;
        t[rson].tag = true;
        t[lson].val = 0;
        t[rson].val = 0;
    }
}
void build(int u, int l, int r) {
    t[u] = {l, r};
    if (l == r) {
        t[u].val = 1;
        t[u].tag = false;
        return;
    }
    int mid = l + r >> 1;
    build(lson, l, mid);
    build(rson, mid + 1, r);
    pushup(u);
}

void update(int u, int l, int r, int k) {
    if (l <= t[u].l &&  t[u].r <= r)  {
        t[u].tag = true;
        t[u].val = k;
        return;
    } 
    pushdown(u);
    int mid = t[u].l + t[u].r >> 1;
    if (l <= mid) update(lson, l, r, k); 
    if (r > mid) update(rson, l, r, k);
    pushup(u);

}

int query(int u, int l, int r) {
    if (l <= t[u].l && r >= t[u].r) return t[u].val;
    int mid = t[u].l + t[u].r >> 1;
    pushdown(u);
    int ans1, ans2;
    if (l <= mid) ans1 = query(lson, l, mid);
    if (r > mid) ans2 = query(rson, mid + 1, r);
    return ans1 + ans2;
}
int main(){
    cin >> L >> m;
    build(1, 1, L + 1);
    for (int i = 1; i <= m; i ++) {
        int x, y;
        cin >> x >> y;
        update(1, x+1, y+1, 0);
    }
    cout << query(1, 1, L + 1);
    return 0;
}