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);    }};