307. Range Sum Query - Mutable

class NumArray {

    private int[] segmentTree;
    private int n;

    public NumArray(int[] nums) {
        n = nums.length;
        segmentTree = new int[2 * n];
        buildSegmentTree(nums);
    }

    private void buildSegmentTree(int[] nums) {
        
        for (int i = 0; i < n; i++) {
            segmentTree[i + n] = nums[i];
        }
        
        for (int i = n - 1; i > 0; --i) {
            segmentTree[i] = segmentTree[2 * i] + segmentTree[2 * i + 1];
        }
    }

    public void update(int index, int val) {
        
        index += n;
        segmentTree[index] = val;
        
        while (index > 1) {
            index /= 2;
            segmentTree[index] = segmentTree[2 * index] + segmentTree[2 * index + 1];
        }
    }

    public int sumRange(int left, int right) {
        int sum = 0;
        left += n;
        right += n;
        while (left <= right) {
            if ((left % 2) == 1) {
                sum += segmentTree[left];
                left++;
            }
            if ((right % 2) == 0) {
                sum += segmentTree[right];
                right--;
            }
            left /= 2;
            right /= 2;
        }
        return sum;
    }
}