題目

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
     /  \
    5   -3
   / \    \
  3   2   11
 / \   \
3  -2   1

Return 3. The paths that sum to 8 are:

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

解題思路

遍歷二叉樹所有節點（任何遍歷方式）
對每個節點，計算他和子節點組成的路徑的節點值的和是否等于目標值，如果是，則路徑的值加1

代碼

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
var ret int

func downTravel(t *TreeNode, sumNeed, sumGet int)  {
    if nil == t {
        return
    }
   
    sumGet += t.Val
    if sumGet == sumNeed {
        ret++
    }
    
    downTravel(t.Left, sumNeed, sumGet)
    downTravel(t.Right, sumNeed, sumGet)
}

func preOrderTravel(t *TreeNode, sum int)  {
    if nil == t {
        return
    }

    downTravel(t, sum, 0)

    preOrderTravel(t.Left, sum)
    preOrderTravel(t.Right, sum)
}

func pathSum(root *TreeNode, sum int) int {
    ret = 0
    preOrderTravel(root, sum)

    return ret
}