LeetCode-Range Sum Query

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

The update(i, val) function modifies nums by updating the element at index i to val.

Example:

Given nums = [1, 3, 5]

sumRange(0, 2) -> 9
update(1, 2)
sumRange(0, 2) -> 8

Note:

1. The array is only modifiable by the update function.
2. You may assume the number of calls to update and sumRange function is distributed evenly.

public class NumArray {
        private int[] tree;
        private int[] num;

        public NumArray(int[] nums) {
            tree = new int[nums.length];
            num = new int[nums.length];
            
            Arrays.fill(tree,0);
            Arrays.fill(num,0);
            for (int i=0;i<nums.length;i++){
                update(i,nums[i]);
            }
        }

        void update(int i, int val) {
            int delta = val-num[i];
            num[i] = val;
            int ind = i+1;

            while (ind<=tree.length){
                tree[ind-1] += delta;
                ind += ind & (-ind);
            }

        }

        private int sum(int i){
            int ind = i+1;
            int sum = 0;

            while (ind>0){
                sum += tree[ind-1];                
                ind -= ind & (-ind);
            }
            return sum;
        }

        public int sumRange(int i, int j) {
            int a = sum(j);
            int b = sum(i-1);
            return a-b;
        }
        
        
    }


// Your NumArray object will be instantiated and called as such:
// NumArray numArray = new NumArray(nums);
// numArray.sumRange(0, 1);
// numArray.update(1, 10);
// numArray.sumRange(1, 2);

Solution: it is an entry-level problem about Binary Indexed Tree.

Check BIT out: http://www.geeksforgeeks.org/binary-indexed-tree-or-fenwick-tree-2/