Path Sum III

Description

You are given a binary tree in which each node contains an integer value.

Find the number of paths that sum to a given value.

The path does not need to start or end at the root or a leaf, but it must go downwards (traveling only from parent nodes to child nodes).

The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.

Example:

root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8
  10
 /  \
<b>5</b>   <b>-3</b>

/ </b>    </b>
3   2   11
/ \   </b>
3  -2   1

Return 3. The paths that sum to 8 are:

5 -> 3
5 -> 2 -> 1
-3 -> 11

Solution(javascript)

/** Prefix Sum 注意这里的顺序
 * @param {TreeNode} root
 * @param {number} sum
 * @return {number}
 */
const pathSum = function (root, sum) {
  let count = 0
  const map  = {0: 1}
  const aux = (node, current) => {
    if (!node) {
      return
    }
    const next = current + node.val
    if (map[next - sum] > 0) {
      count += map[next - sum]
    }
    map[next] = (map[next] || 0) + 1
    aux(node.left, current + node.val)
    aux(node.right, current + node.val)
    map[next] -= 1
  }
  aux(root, 0)
  return count
}