Binary Indexed Tree(Fenwick Tree)

https://www.geeksforgeeks.org/binary-indexed-tree-or-fenwick-tree-2/

首先需要了解负数的二进制表示：负数的二进制表示是取反然后+1 

def binstr(x):
    ss = []
    for i in range(32):
        ss.append((x >> i) & 0x1)
    ss = ss[::-1]
    return ''.join(map(str, ss))

for x in range(10, 16, 2):
    print('bin({:>4s}) = {}'.format(str(x), binstr(x)))
    y = -x
    print('bin({:>4s}) = {}'.format(str(y), binstr(y)))

bin(  10) = 00000000000000000000000000001010
bin( -10) = 11111111111111111111111111110110
bin(  12) = 00000000000000000000000000001100
bin( -12) = 11111111111111111111111111110100
bin(  14) = 00000000000000000000000000001110
bin( -14) = 11111111111111111111111111110010

然后了解一下index & (-index) 这个操作的含义：找到lsb（找到最低位置的1） 

print(12 & (-12))
print(binstr(12))

4
00000000000000000000000000001100

最下面图里面给出的 i & -i 的操作解释很好：

  1. 假设i = a 1 0k 的话
  2. ~i = (~a) 0 1k
  3. -i = (~a) 1 0k
  4. i & -i = 1 0 0k

UPDATE@201809: 最下面的配图可能有助于理解这棵树的组织

Binary Indexed Tree实际上进行分段存储，每段长度都是2 ** k - 1，每段里面可以继续拆分：

以31为例：31 = 16 + 8 + 4 + 2 + 1. 那么可以分为五段： 

1. B[16] = nums[1] + … nums[16]
2. B[24] = nums[17] + … nums[24]
3. B[28] = nums[25] + … nums[28]
4. B[30] = nums[29] + nums[30]
5. B[31] = nums[31]

这个过程就是不断地 index -= (index & -index).

然后考虑一下对于nums[index]可能会影响到哪些B呢？假设len(nums) == 32, index = 29. 从上面例子看到 

1. B[30] = nums[29] + nums[30]
2. B[32] = nums[1] + … nums[32]

所以这个过程和上面相反，index += (index & -index). 

/*            n  --> No. of elements present in input array.
    BITree[0..n] --> Array that represents Binary Indexed Tree.
    arr[0..n-1]  --> Input array for whic prefix sum is evaluated. */

// Returns sum of arr[0..index]. This function assumes
// that the array is preprocessed and partial sums of
// array elements are stored in BITree[].
int getSum(int BITree[], int index)
{
    int sum = 0; // Iniialize result

    // index in BITree[] is 1 more than the index in arr[]
    index = index + 1;

    // Traverse ancestors of BITree[index]
    while (index>0)
    {
        // Add current element of BITree to sum
        sum += BITree[index];

        // Move index to parent node in getSum View
        index -= index & (-index);
    }
    return sum;
}

// Updates a node in Binary Index Tree (BITree) at given index
// in BITree.  The given value 'val' is added to BITree[i] and
// all of its ancestors in tree.
void updateBIT(int BITree[], int n, int index, int val)
{
    // index in BITree[] is 1 more than the index in arr[]
    index = index + 1;

    // Traverse all ancestors and add 'val'
    while (index <= n)
    {
       // Add 'val' to current node of BI Tree
       BITree[index] += val;

       // Update index to that of parent in update View
       index += index & (-index);
    }
}

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