线段树查询2

对于一个数组，我们可以对其建立一棵

线段树

, 每个结点存储一个额外的值

count

来代表这个结点所指代的数组区间内的元素个数.
(数组中并不一定每个位置上都有元素)

实现一个

query

的方法，该方法接受三个参数

root

,

start

和

end

,
分别代表线段树的根节点和需要查询的区间，找到数组中在区间[start, end]内的元素个数。

/**
* Definition of SegmentTreeNode:
* class SegmentTreeNode {
* public:
*     int start, end, count;
*     SegmentTreeNode *left, *right;
*     SegmentTreeNode(int start, int end, int count) {
*         this->start = start;
*         this->end = end;
*         this->count = count;
*         this->left = this->right = NULL;
*     }
* }
*/
class Solution {
public:
/**
*@param root, start, end: The root of segment tree and
*                         an segment / interval
*@return: The count number in the interval [start, end]
*/
int query(SegmentTreeNode *root, int start, int end) {
if(root==NULL||start>end||start>root->end||end<root->start) return 0;
int mid=root->start+(root->end - root->start)/2;

if(start==root->start&&end==root->end) return root->count;
if(root->start==root->end) return root->count;
if(start>mid) return query(root->right,start,end);
if(end<=mid)  return query(root->left,start,end);
return query(root->left,start,mid)+query(root->right,mid+1,end);

}
};