lintcode-Segment Tree Query II

/**
 * 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) {
        // write your code here
        if(root == NULL) { return 0; }
        if(start > end || start > root->end || end < root->start) { return 0; }
        if(start <= root->start && end >= root->end) { return root->count; }
        
        int mid = (root->start + root->end) / 2;
        
        int left = query(root->left, start, min(mid, end));
        int right = query(root->right, max(mid, start), end);
        
        return left+right;
    }
};