public SegTree buildTree(int nums[], int i, int j) {
if (j < i) return null;
SegTree node = new SegTree(i, j);
if (i == j) {
node.sum = nums[i];
return node;
}
int mid = (i + j) / 2;
node.left = buildTree(nums, i, mid);
node.right = buildTree(nums, mid + 1, j);
if (node.left != null) node.sum += node.left.sum;
if (node.right != null) node.sum += node.right.sum;
return node;
}
