golang力扣leetcode 124. 二叉树中的最大路径和

package main
import "math"
type TreeNode struct {
  Val   int
  Left  *TreeNode
  Right *TreeNode
}
func maxPathSum(root *TreeNode) int {
  maxSum := math.MinInt32
  var dfs func(root *TreeNode) int
  dfs = func(root *TreeNode) int {
    if root == nil {
      return 0
    }
    left := dfs(root.Left)
    right := dfs(root.Right)
    //左+根+右 情况
    all := left + root.Val + right
    //更新到全局变量
    maxSum = max(maxSum, all)
    //max(左，右)+根 --->单边 情况
    half := root.Val + max(left, right)
    //主要是为了应为负数的情况，既然是负数，还不如不加
    return max(half, 0)
  }
  dfs(root)
  return maxSum
}
func max(a, b int) int {
  if a > b {
    return a
  }
  return b
}