线段树 C++实现 树形式

网上看了一圈，看到几个都是用数组实现的
我用树结构重写了一遍

#ifndef SEGMENTTREE_H
#define SEGMENTTREE_H
#include <vector>

template<typename T>
class SegmentTree {
   public:
      SegmentTree(std::vector<T> &a) {
            int N = a.size();
            this->a = a;
            this->root = new _Node();
            this->_build(0, N-1, this->root);
      };

      virtual ~SegmentTree() {
            delete this->root;
      };

      void updateElement(T value, int index) {
            this->_updateSegment(value, index, index, this->root);
      }

      void updateSegment(T c, int l, int r) {
            this->_updateSegment(c, l, r, this->root);
      };

      T querySegment(int l, int r) {
            return this->_querySegment(l,r, this->root);
      };
   private:
      struct _Node {
            _Node* rchild;
            _Node* lchild;
            int l, r, lazy;
            T sum;

            _Node() {
                  this->rchild = nullptr;
                  this->lchild = nullptr;
                  this->l = -1;
                  this->r = -1;
                  this->lazy = -1;
                  this->sum = 0;
            }

            ~_Node() {
                  delete this->rchild;
                  delete this->lchild;
            }

            int mid() {
                  return (l+r) >> 1;
            };

            void _pushUp() {
                  this->sum = this->rchild->sum + this->lchild->sum;
            };

            void _pushDown(int length) {
                  // lazy load
                  if(this->lazy) {
                        // push down the lazy value into children
                        this->lchild->lazy += this->lazy;
                        this->rchild->lazy += this->lazy;
                        this->lchild->sum += this->lazy * (length-(length>>1));
                        this->rchild->sum += this->lazy * (length>>1);
                        this->lazy = 0;
                  }
            };
      };
      _Node *root;

      std::vector<T> a;

      void _build(int l, int r, _Node *rt) {
            rt->l = l;
            rt->r = r;
            rt->lazy = 0;
            rt->rchild = new _Node();
            rt->lchild = new _Node();

            if(l == r) {
                  rt->sum = this->a[l];
                  return ;
            }

            int m = rt->mid();
            this->_build(l, m, rt->lchild);
            this->_build(m+1, r, rt->rchild);
            rt->_pushUp();
      };

      void _updateSegment(T value, int l, int r, _Node *rt) {
            if(rt->l == l && rt->r == r) {
                  rt->lazy += value;
                  rt->sum += value * (r-l+1);
                  return;
            }

            if(rt->l == rt->r) return ;

            rt->_pushDown(rt->r - rt->l + 1);

            int m = rt->mid();
            if(r <= m) this->_updateSegment(value, l, r, rt->lchild);
            else if(l > m) this->_updateSegment(value, l, r, rt->rchild);
            else {
                  this->_updateSegment(value, l, m, rt->lchild);
                  this->_updateSegment(value, m+1, r, rt->rchild);
            }

            rt->_pushUp();
      };

      T _querySegment(int l, int r, _Node *rt) {
            if(l == rt->l && r == rt->r) {
                  return rt->sum;
            }

            rt->_pushDown(rt->r - rt->l + 1);

            T ans = 0;
            int m = rt->mid();
            if(r <= m) ans += this->_querySegment(l, r, rt->lchild);
            else if(l > m) ans += this->_querySegment(l, r, rt->rchild);
            else {
                  ans += this->_querySegment(l, m, rt->lchild);
                  ans += this->_querySegment(m+1, r, rt->rchild);
            }
            return ans;
      }
};
#endif // SEGMENTTREE_H