# Interval Sum

LintCode-206.Interval Sum

Given an integer array (index from 0 to n-1, where n is the size of this array), and an query list. Each query has two integers [start, end]. For each query, calculate the sum number between index start and end in the given array, return the result list.

Example

For array [1,2,7,8,5], and queries [(0,4),(1,2),(2,4)], return [23,9,20]

Challenge

O(logN) time for each query

``````/**
* Definition of Interval:
* public classs Interval {
*     int start, end;
*     Interval(int start, int end) {
*         this.start = start;
*         this.end = end;
*     }
*/
public class Solution {
/**
*@param A, queries: Given an integer array and an query list
*@return: The result list
*/
public ArrayList<Long> intervalSum(int[] A,
ArrayList<Interval> queries) {
if (A == null || A.length == 0
|| queries == null || queries.size() == 0) {
return new ArrayList<Long>();
}

SegmentTreeNode root = buildTree(0, A.length - 1, A);

ArrayList<Long> result = new ArrayList<Long>();
for (Interval interval : queries) {
long sum = query(root, interval.start, interval.end);
}

return result;
}

long query(SegmentTreeNode node, int start, int end) {
if (node == null) {
return 0;
}

if (start <= node.start && end >= node.end) {
return node.sum;
}

int middle = node.start + (node.end - node.start) / 2;

if (end <= middle) {
return query(node.left, start, end);
} else if (start > middle) {
return query(node.right, start, end);
} else {
return query(node.left, start, middle) + query(node.right, middle + 1, end);
}
}

SegmentTreeNode buildTree(int start, int end, int[] A) {
if (start > end) {
return null;
}

SegmentTreeNode node = new SegmentTreeNode(start, end);

if (start != end) {
int middle = start + (end - start) / 2;

node.left = buildTree(start, middle, A);
node.right = buildTree(middle + 1, end, A);

node.sum = node.left.sum + node.right.sum;
} else {
node.sum = A[start];
}

return node;
}
}

class SegmentTreeNode {
int start, end;
long sum;
SegmentTreeNode left;
SegmentTreeNode right;

public SegmentTreeNode(int start, int end) {
this.start = start;
this.end = end;
this.sum = 0;
this.left = this.right = null;
}
}
``````

Hope this helps,
Michael