4. 树形DP

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func rob(root *TreeNode) int {
	return Help(root)
}
//缓存很重要，可以存很多子问题的值
var m = map[*TreeNode]int{}
func Help(root *TreeNode) int{
	if root == nil{
		return 0
	}
	if v,ok := m[root];ok{
		return v
	}
	res,do,not_do := 0,root.Val,0
	//do / not_do
	if root.Right != nil{
		do +=  Help(root.Right.Right) + Help(root.Right.Left)
	}
	if root.Left != nil{
		do += Help(root.Left.Left) + Help(root.Left.Right)
	}
	not_do += Help(root.Left)+Help(root.Right)
	res = MAX(do,not_do)
	m[root] = res
	return res
}

func MAX(i,j int) int{
	if i<j{
		return j
	}else{
		return i
	}
}