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 */