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);
result.add(sum);
}
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