LeetCode 307

class NumArray {
    int[] tr;
    int[] nums;
    int n;
    
    private int lowbit(int x) {
        return x & -x;
    }
    
    private void add(int x, int v) {
        for (int i = x; i <= n; i += lowbit(i)) 
            tr[i] += v;        
    }
    
    private int query(int x) {
        int res = 0;
        for (int i = x; i > 0; i -= lowbit(i))
            res += tr[i];
        return res;
    }
    
    
    public NumArray(int[] nums) {
        n = nums.length;
        tr = new int[n + 1];
        this.nums = nums;
        for (int i = 1; i <= n; i++) {
            add(i, nums[i - 1]);
        }
    }
    
    public void update(int index, int val) {
        add(index + 1, val - nums[index]);
        nums[index] = val;
    }
    
    public int sumRange(int left, int right) {
        return query(right + 1) - query(left);
    }
}

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * obj.update(index,val);
 * int param_2 = obj.sumRange(left,right);
 */