思路：
动态开点线段树，这里维护了区间和和区间赋值。
实现：
1 class SegmentTree {
2 public:
3 int N = (int)1e9;
4 class Node {
5 public:
6 // ls 和 rs 分别代表当前区间的左右子节点
7 Node*ls=nullptr, *rs=nullptr;
8 // val 代表当前区间的最大高度，add 为懒标记
9 int val=0, add=0;
10 Node(){}
11 };
12 Node*root = nullptr;
13 SegmentTree(int n){
14 root=new Node();
15 N=n;
16 }
17 void update(int l,int r,int v){
18 _update(root,0,N,l,r,v);
19 }
20 void _update(Node*node, int lc, int rc, int l, int r, int v) {
21 if (l <= lc && rc <= r) {
22 node->add = v;
23 node->val = v*(rc-lc+1);
24 return ;
25 }
26 pushdown(node,lc,rc);
27 int mid = lc + rc >> 1;
28 if (l <= mid) _update(node->ls, lc, mid, l, r, v);
29 if (r > mid) _update(node->rs, mid + 1, rc, l, r, v);
30 pushup(node);
31 }
32 int query(int l,int r){
33 return _query(root,0,N,l,r);
34 }
35 int _query(Node*node, int lc, int rc, int l, int r) {
36 if (l <= lc && rc <= r) return node->val;
37 pushdown(node,lc,rc);
38 int mid = lc + rc >> 1, ans = 0;
39 if (l <= mid) ans = _query(node->ls, lc, mid, l, r);
40 if (r > mid) ans += _query(node->rs, mid + 1, rc, l, r);
41 return ans;
42 }
43 void pushdown(Node*node,int lc,int rc) {
44 if (node->ls == nullptr) node->ls = new Node();
45 if (node->rs == nullptr) node->rs = new Node();
46 if (node->add == 0) return ;
47 int mid=lc+rc>>1;
48 node->ls->add = node->add; node->rs->add = node->add;
49 node->ls->val = node->add*(mid-lc+1); node->rs->val = node->add*(rc-mid);
50 node->add = 0;
51 }
52 void pushup(Node*node) {
53 node->val = node->ls->val + node->rs->val;
54 }
55
56 };
57 class CountIntervals {
58 public:
59 SegmentTree*st=NULL;
60 CountIntervals() {
61 st=new SegmentTree(1e9);
62
63 }
64
65 void add(int left, int right) {
66 st->update(left,right,1);
67
68
69 }
70
71 int count() {
72 return st->query(0,1e9);
73
74 }
75 };
76
77 /**
78 * Your CountIntervals object will be instantiated and called as such:
79 * CountIntervals* obj = new CountIntervals();
80 * obj->add(left,right);
81 * int param_2 = obj->count();
82 */